London's tax take is equivalent to 45.2% of its GDP and public spending eqivalent to only 34.9% of its GDP.

Compare with some regions:

NW: 37.5%, 55.9% (ie big deficit)
NE: 29.7%, 61.9% (bigger deficit still)
Northern Ireland: 27.7%, 67% (even bigger deficit)

Large chunks of the UK are unsustainable.

Boris can make a fair argument because if it came down to every region keeping its taxes and paying for its costs, then London would be better off than everywhere else because the economic activity in the UK subsidises everywhere else. I remember seeing an infographic when I was at university which was a 3D map of Europe where the higher points showed where economic activity was highest, all European countries had a large spike up at their capital city but the UK's was frightening, it was basically a flat country, with a ridge in the south east going up to a huge pole in London.

However there is a key fault with Boris' argument in terms of 'urban giantism' which is a problem that blights a lot of developing countries where they have one huge city that dominates their country's economy - Santiago in Chile, Tehran in Iran etc, if you want to set up a business in these countries you have to be in the giant city because thats where the market is, which draws more and more businesses towards it, and hence more and more people come to follow, it becomes a giant sucking plughole that everybody is drawn into if they want to get a job or set up any kind of business, over time if you set up business elsewhere and rely on any sort of suppliers you can't get them locally because they are all in the capital. The result is you get a poor rest of the country and a capital city with enormous congestion problems, gridlocked roads, pollution and very poor standard of housing. London already has some of these problems and if you go down Boris' path it will end up like those cities.

Interesting that London is the part of the UK that has the most immigration and where multiculturalism is most advanced and this is the part of the UK that pays its own way. Is the problem that the rest of the UK is backward thinking and needs to open its doors to more immigrants to try and replicate London's success?

Possibly.

More likely that it draws in graduates, etc from other parts of the UK. If London were to pay transfer fees for people to the regions from whence they came then maybe it would be seen as being subsidised? Certainly, an awful lot of business people and high earners I've come across over the years in London were not born and bred in the city.

But yet large parts of the UK can feed itself locally London can't.
Perhaps the North of the UK should stop supplying London with food then Boris and his chums can eat all the money they clearly love so much.

They should be sold to the highest bidder. The Chinese, say.

Cities of many millions of people are notoriously self sustaining and don't at all rely on, for instance, food, water, electricity or other goods and services produced elsewhere. Human resources in a city like London - renowned the world over for everyone in it being born there - don't even enter the equation, so the good health and education in childhood of people not born there shouldn't be factored in to its success. Those roads outside the city, even if pointed in the right general direction, don't actually go anywhere relevant. With no goods or services being moved along them they are just a distraction from what by rights should be the kind of countryside you could have a picnic in when you want to take a break from that exaggerated sense of entitlement.

London has carried the burden of the rest of the country for too long.