(1) They think nowhere else in Britain has culture or entertainment that is worthwhile.
(2) They think they live in the most multiethnic city in the world. Leaving aside whether that is something to celebrate or not - and I take London's ethnic diversity as merely something value-neutral - it is not objectively true anyway. New York is incomparably more multiethnic than London: it has fewer South Asians, but many more Jews, East Asians, Blacks and in particular Latin Americans. Its White population is also at least equally varied as London's. What's more, certain other cities such as Paris, Toronto, Vancouver, Los Angeles, Houston, Sydney, Melbourne and maybe even some cities in Brazil at least rival if not exceed London for ethnic diversity too.
(3) They think they live in the most sexually-liberated and open-minded city in Britain, if not the world. Insofar as that was even remotely true in the past, ironically in part thanks to the aforementioned ethnic diversity of the city, that is certainly no longer the case. In fact, a recent British Social Attitudes Survey showed that Londoners are actually the least tolerant British people when it comes to homosexuality and pre-marital sex, saying that 'religion' is a major explanation behind this.
(4) They won't countenance genuine decentralisation and redistribution of economic and political power beyond a very superficial and minor level. This is especially true when it comes to finance and the dominance of the City.
(5) The overwhelming majority of the few transport upgrades and projects Britain has are concentrated in the capital, such as Crossrail. By contrast, even rail electrification has been refused to Wales west of Cardiff and Southwest England south of Bristol.