Climb to the top of the Monument and you can’t see one Ukip councillor. Climb to the top of the Shard and the horizon is crowded with them. What’s wrong? In the week after Ukip won its first MP and its poll support hit 25 per cent, the puzzle remains. Why is Greater London a no-go for Ukip? It seems immune to the charms of Nigel Farage as he rampages across Britain’s political landscape.

The answer is that London is different, and gets more so every year. It is different in precisely the way Farage deplores. To him London is hardly England at all. It is cosmopolis, a tax haven for rich refugees, a money-laundering bolt hole, a gilded beach onto which is washed up the flotsam of Europe and the world. Is this the England you want, asks Farage, as he slams his pint down on the bar and looks in vain for someone to whom he can speak English.

London votes Ukip in single figures, barely a third of its support elsewhere. Ukip actually lost London seats at last May’s local elections, at the time of its triumph in the Euro-elections. The reason is indeed Farage’s fear. The mice are not going to vote for cats.

Forty per cent of Londoners were not born in Britain. A third of them are non-white. Take a weekend walk through Regent’s Park or Hyde Park, or a stroll up Edgware Road or through Knightsbridge and you might indeed wonder where you are. Kensington and Chelsea is now depopulating at such a rate that it is losing English people faster than it’s gaining foreigners.

To find substantial Ukip support you have to go to fringe boroughs such as Havering and Bexley. The party’s current targets are Rochester and Strood, and South Thanet, in classic working-class Kent. One of its London leaders, Suzanne Evans of Merton, even took Farage’s argument further. In an unguarded moment she blamed her own defeat in May on the capital’s “cultured, media-savvy, well-educated” voters. Smooth metropolitans, she said, did not get “the heartache” over immigration felt by provincial electors. They were haves to Ukip’s have-nots.

This is mostly rubbish. The Mayor, Boris Johnson, once a valiant defender of London’s newcomers, is now blatantly switching as his parliamentary career beckons. He suggested to the BBC at the weekend that it was “a mistake to take the brakes off” immigration. He called border control “the number one thing that we need”. But even he knows that London’s success depends on defying what Farage espouses. It is the diversity capital of Europe.

London feasts on a population influx that Ukip claims is rotting England’s cultural core. It welcomes the rest of the world — including the rest of Britain — with open arms. In return the world contributes its wealth, its talent, its vitality and its youth. English may be the first language of ever fewer citizens but it is the second language of all. London is a free market in humanity.

The capital may have its quarrels with the rest of Europe — staying out of the euro has been vastly to its advantage — but there is a solid cultural chain linking it to cities such as Paris, Berlin, Amsterdam and Barcelona. London must be the least xenophobic city on Earth. As for Farage’s implied complaint that its cafés and bars are crowded with a cosmopolitan, public-school establishment, move over Nigel, Huguenot late of Dulwich College, and make room for the rest of us.

For all that, London should tread carefully. Ukip will blow itself out. Its central policy, an EU referendum, is a self-destruct button. If the vote goes for the EU, Farage himself admits the game is over for him. If it goes against, all political parties are in the same boat. In the meantime all Ukip can do is devastate the party it most resembles, the Tories, in the south. It is what the Social Democrats did to Labour in 1981-83. Even Farage’s argument with David Cameron over the referendum is now a matter of dates.

More serious is the wider appeal that Ukip makes to a provincial electorate. It is the message of being the party of not-London. Farage takes a leaf from the book of Scotland’s Alex Salmond, harping constantly on about “the London political establishment” which has “fooled the people for too long”.

He attacks London for its smugness, its aloofness, its not caring for the rest of Britain. He attacks its big-government grandees and its tax- dodgers, sucking ever more of the nation’s wealth to its capacious vaults. Give London a punch on the nose, says Farage, and vote Ukip. It is good politics.

One day Ukip may wane but the “case against London” will not. The capital has been the chief winner from a centralised UK. It grabs the lion’s share of capital spending, from Crossrail and the Olympics to HS2, the arts and university research. Its local taxes are ludicrously low, its tax reliefs absurdly generous. The greed of its property market is emptying whole neighbourhoods for “buy-to-leave”. It is a grotesquely unequal city.

That centralism may be coming to an end. London may be the nation’s political powerhouse but its arrogance breeds resentment elsewhere. It is big and rich but it is not the whole nation — and the nation can easily outvote it.

We might joke that, if push came to shove, London could declare UDI and take itself off into limbo. It could become a Monaco or a Liechtenstein, a plutocratic Ruritania under the munificent rule of King Boris of Zog. But this is not sensible.

Ukip’s lack of appeal in London may comfort metro-liberals but it is a warning. What appeals to Farage also appeals to millions of provincial voters. One day London may need the rest of Britain. It should not forget it.