(Fixes typo in 'sixth', paragraph 3)
* Farage says PM's EU vote pledge a game changer
* Britons turning against EU over immigration, says Farage
* Says European Union heading for disaster
By Andrew Osborn and Guy Faulconbridge
LONDON, Jan 29 (Reuters) - Britain's withdrawal from the European Union "within a few years" is a certainty as no government will be able to resist demands for an exit from a population incensed by the arrival of low-wage migrants, the leader of the UK Independence Party said.
Nigel Farage, who is siphoning off voters from Prime Minister David Cameron's Conservatives by attacking EU bureaucracy and immigration from eastern Europe, said it was the dramatic rise of his own party that had pressured the premier into promising last week what would be a historic EU referendum.
"Cameron's speech was the moment when the debate on Europe changed," a combative Farage told Reuters, calling it UKIP's "greatest victory to date". Dismissing the idea Cameron had outflanked the anti-EU lobby, he said the reverse was true: Cameron had "let the genie out of the bottle", making "Brexit" - the EU exit of the world's sixth largest economy - certain.
Speaking days after Cameron promised to renegotiate the terms of Britain's 40-year-old membership of the bloc and to hold an "in-out" referendum if re-elected in 2015, Farage said Cameron would probably lose office and, even if he won, would be unable to persuade other EU leaders to amend treaty agreements.
"There is no substantial renegotiation to be had," said Farage. "He'll get nowhere." In any event, the prime minister, who says he wants to stay in a revamped European Union, could not be trusted to keep his referendum promise, he added, noting abandoned talk of a vote on the Lisbon Treaty of 2007.
But with polls showing a slim majority want to cut Britain loose from an expanding European political system, a referendum was not far off, Farage forecast. Even the Labour party, which opposes Cameron's in-out referendum, would, if it took power, be forced by the public to let Britons vote to leave, he argued.
It is a prospect that clearly enthuses Farage, a 48-year-old former metals trader in the City of London who has seen UKIP multiply its support in the six years since Cameron dismissed it as "a bunch of ... fruitcakes and loonies and closet racists".
"I see opportunity. We would have the potential to do so much more for this country," he said at the central London headquarters of a party which has lately polled as much as 16 percent, up from 3 percent at the last election in 2010.
"We'd have the opportunity - through winning back the right to have global trade agreements - to make us much more of a global player than a European player," he said.
Farage's sharp tongue - pro-EU evangelists were "idiots", he said - and a populist touch that includes a fondness for a smoke and a drink, have made him a familiar figure to voters. He is credited with bringing a new professionalism to a 20-year-old party long dismissed as part of a chaotic, far-right fringe.
Attacked from the left as a "dangerous man" and "political poison", he imagines a Britain thriving outside the EU, a picture that has struck a chord with voters alarmed by the way the debt crisis in the euro zone has hobbled their own economy and at poor immigrants arriving freely from eastern Europe.
EU red tape, Farage said, could be junked along with the bloc's social market model, heavy on regulation and welfare provision. That, he said, would make Britain a more attractive destination for manufacturers. It would also be free to strike bilateral trade deals and to deregulate its own economy.
"I don't want to turn my back on Europe but it's not the future," said Farage. "The future is the emerging world."
While many businessmen support a renegotiation of Britain's EU membership, they have also warned that years of doubt over Britain's EU membership would damage the $2.5 trillion economy and hinder investment. Farage disagrees.
"Europe has a demographic time bomb," he said.
"It's stuck with the idea of a social market model ... which means it's falling behind in competitiveness terms ... and it has a euro zone crisis which it is determined not to admit defeat on which means a decade of agony."
Cameron accepts much of that analysis but argues that London can distance itself from the problems while staying in the EU.
His demand for change in Brussels and pledge of a referendum seem to have gone down well with voters. One poll has suggested a swing of up to four percent from UKIP to the Conservatives.
For those keen to vote No to Europe, a vote for Cameron in 2015 may seem a better bet, since voting UKIP is more likely to bring a Labour government not committed to an EU referendum.
But Farage dismisses suggestions Cameron has stolen his thunder and deprived UKIP of its distinctive appeal: "He's let us down on this very same promise once before," he said of a Conservative plan for an EU vote. "He's used up so much trust."
UKIP was still the only party that favoured complete and immediate withdrawal from the EU, he said: "The Conservative party is wedded to political union of the European Union.
"They believe in it, they have done for over 50 years. He's actually calling for deeper integration."
That is a complaint that has found favour in Conservative heartlands across the wealthy suburbs and rural areas of southern England. Though a winner-takes-all constituency voting system means UKIP has no seats in the London parliament, it has 12 in the European legislature in Brussels.
MAN OF CONTRASTS
Once shunned by Britain's mainstream media, Farage rubbishes long-standing accusations his party are "the BNP in blazers" - a middle-class, golfing version of skinhead racists in the British National Party - and is now a regular on political talk shows.
In sharp pinstripe suit, black fedora and colourful silk tie, he dresses like the City trader he once was.
Fast-talking, he peppers his speech with jokes and the odd expletive and professes that though politics is a deadly serious business "it doesn't mean you can't have a bit of fun too".
It is a routine he says he has perfected as "a street scrapper" in many a debate in pubs up and down the country.
Married and a father of four, Farage has survived being run down by a car - when he admits he was drunk - a plane crash during an election stunt, and testicular cancer, but said his lust for "living life" was undimmed.
He smokes, enjoys British beer and French wine, bets on horses, likes sea fishing and some years ago shrugged off tabloid headlines about a night with a young Latvian woman.
Farage is a paradoxical figure. Though frequently derided as "a little Englander" - a jibe at his isolationist views - he is married to a German. And though he says he hates everything about the EU, he is a lawmaker in the European Parliament.
Even the location of his London office - Europe House, a building that also houses the British headquarters of the European Commission - is paradoxical, as is the fact that he uses EU funds to campaign against the bloc.
Some prominent business leaders have warned that the prospect of a referendum on the country's EU membership will create years of uncertainty over trading rules that will scare off foreign investors or, as Labour leader Ed Miliband said, hang a "closed for business" sign over the island nation.
Leaving the EU itself, Britain's biggest trading partner would be folly, they have said, and force Britain to renegotiate a free trade agreement from a position of weakness.
But Farage, who argues that trade with the EU has been routinely "overstated", dismissed such fears, saying a British EU exit "wouldn't make much difference" to trade.
"Do you honestly think that Angela Merkel is going to pick up the phone to the chief executive of Mercedes and say we're very sorry but as a result of putting tariffs on British goods they've just slapped tariffs on you selling your cars in Britain?" he asked with a laugh. "They'd go berserk."
The idea that London would lose its status as Europe's financial centre and forfeit the right to trade the single currency if Britain left was also fantasy, he argued.
"Governments can't control markets," he said. "If they really wanted to prohibit euro trading outside the EU they could do that ... if they want it to become about as rich as Congo in a decade. But they're not going to do that."
Farage told Reuters the biggest reason UKIP was attracting more supporters was because it advocated ending "open door" immigration and curbing the right of other EU nationals to work, settle and claim social security payments in Britain.
"Immigration is a very, very key reason for people voting for us," he said, arguing that local communities had been rendered "unrecognisable" by migrant workers in the last decade.
"The levels of division and enmity that have been created within those communities by government policy a propos open borders with the EU is something that people are really, really angry about," he said.
His immediate campaign goal is to lobby against Romanians and Bulgarians getting full rights to work in Britain next year, something they will obtain because of EU freedom of movement rules, seven years after their countries joined the bloc.
Farage strongly rejects any suggestion he is a racist and said he favoured an Australian-style selective immigration policy that chose people because of their skills.
But allegations his party is racist have dogged him.
A local authority in the north of England took three foster children away from a couple in November, arguing that their support for UKIP was racist, prompting Cameron's office to issue comments suggesting it agreed, at least in part.
Earlier this month, Cameron hinted he did not think Farage should be allowed to take part in TV debates ahead of the next election in 2015, telling a magazine that only parties "that are going to form the government" should be included.
Liberal newspapers in Britain are also sceptical. The Independent published an article in December calling Farage "a dangerous man", his views "political poison", and his party's free-market economic policies "a hard-right wet dream".
UKIP's journey to the political mainstream is far from complete at the ballot box; Britain's first-past-the-post electoral system - which only values a party's support in each constituency rather than nationwide - means it would not win a single parliamentary seat if an election was held today.
And one recent poll, the methodology of which Farage contests, put UKIP support at just seven percent. Most others have, however, estimated its vote at up to 16 percent.
Naming his heroes, Farage listed Robert Peel, a 19th-century Conservative prime minister who defied party aristocrats to end corn tariffs and promote free trade, William Wilberforce, who helped end the slave trade, and Enoch Powell, a leading Conservative ostracised in 1968 after criticising immigration policy in what became known as the "rivers of blood" speech.
What Farage said he admired about them all was that they challenged the status quo, regardless of the consequences.
He didn't spell it out, but it's clear he thinks he is doing the same with Britain and the EU: "The same idiots that are now telling us that unless we stay part of the EU single market we're doomed are the same idiots who told us if we didn't join the euro we'd lose all that business," he said.
"What do they know?" (Editing by Alastair Macdonald)