London doesn't subsidise the UK...UK borrowing subsidises the UK and we all share in paying for that....that's what an annual budget deficit means. Anyway, there are three regions which have income over expenditure surpluses, not just London, the East and South East also have surpluses, although I suspect they also don't have annual accounts which add a share of UK debt, a share of UK defence, the annual maintenance and running of an English Parliament and its dedicated MPs or the whole funding of the infrastructure built in London, so much of which is deemed to be of benefit to the whole UK, even though it isn't, in order that the taxpayer pays a share.
It is unsurprising that London has the highest income in the UK, anyway, because pretty much everything in the UK ends up in London .....but what would go if it became an independent state? Would rUK continue to have the UK/English Parliament there and the Government Cabinet offices/Departments with all the highest paid jobs, and the 18.5% of UK civil servants (78,000+ London taxpayers, many of them highly paid taxpayers) employed there? Would rUK pay a large chunk of the maintenace costs of Westminster, Buck House, Tower of London etc? Would the UK public sector employers who employ around 600,000 people in London still be in London as an independent country? And if London was an independent country outside the EU, would it still have the same reach for selling their financial and other professional services? And would they be able to afford to allow big businesses to dodge tax in London as they currently turn a blind eye to in the UK?
If they could be sure of keeping what they have, then they probably could be independent, but could they keep what they already have without being the Capital of the UK, and sooking doon to London most of the UK companies head offices and their profits to the head offices of the banks? London itself doesn't produce much that is tangible and exportable so I suspect their import/export statistics re food and "things" would struggle as much as England's does now, even with London's exported services and Scotland trade surplus. Independence for London is an interesting idea, though.