https://www.ons.gov.uk/visualisations/dvc2872/fig03/index.ht...
None of that will counted as "public subsidy" but collectively that will run into the billions. All those politicians, civil servants and BBC employees based there are spending UK public money to eat, sleep and travel within the city.
If a place like Liverpool had all that instead of London, then it would be Merseyside mysteriously generating a high surplus.
Much of British transport infrastructure is also designed to feed people into London (although London is less bad for that than Paris which is the hub of nearly all the French rail network.) The Channel Tunnel certainly does. Even the motorway and rail networks of Wales do to some extent.
London has umpteen infrastructure updates whether motorways or underground lines. Also vanity projects such as the Garden Bridge which cost tens of millions without ever being built, or the Wembley Stadium refit which cost than the Scottish Parliament building (but garnered a fraction of the negative press.)
In general, denser cities are more efficient in very measure. It makes sense, you have the same amount of stuff with less space and less materials.
Basically everywhere the way it works is that the non-cities are on perpetual welfare from the cities because the cities have all the economy, but you still have to run miles of roads and sewage into the non-cities.
The Garden Bridge project alone swallowed tens of millions and remained completely unbuilt. That money could have been better spent elsewhere with greater results.
Yes it's true that the rest of the UK could benefit from more investment and that MIGHT make otherwise economically defunct areas slightly less economically defunct. Keyword might. London is a "will", not a might.
If you look at the geography of London it makes for a good port area for bringing goods to and from continental Europe, but it no longer does that like it used to. In the past it was highly dependent on coal from Yorkshire, South Wales etc and materials from elsewhere whether wool or metals. In the 1980s, the London stock market was bankrolled by North Sea oil, mostly in northern English and Scottish waters. Nowadays, London's chief thing is the finance industry and stock market, but that is not location dependent at all, and very easily transferred somewhere else which puts London in a more unsafe position than it realises. Like most major cities, it consumes a great deal of energy and gets its water from elsewhere (and some recycling).