London makes a lot of money because it's dominated by high-return service industries, most famously the financial sector based in the City. It includes relatively little in the way of production, manufacturing and storage, which don't scale as well but are obviously essential to everything else being possible.
The vast majority of London commuters live in Greater London, so its housing is largely provided for by itself. Even if you assumed every human being in the "commuter belt" travelled to London daily, the GLA would still account for ~73% of the population, and this is plainly an absurd assumption.
While London may not contribute as much manufacturing, it still manages ~6% of the UK's manufacturing output. At the same time, just retail in London produces almost as much for the economy as rUK manufacturing.
Your whole thesis is laughable. You seem to be upset that the poorer parts of the nation can trade with London to provide it goods and services. Should you not be upset at the farming, storage and transportation rUK "harvests" from abroad? This dwarfs London's "harvesting" of rUK.
London itself has nothing to do with profits from this trade being unequally distributed. Westminster, sure, but London? No.
Not everything is about GDP. You belittle farming, but without food to eat, Londoners would not be generating a lot of economic output at all.
The original point here was that London generates disproportionate tax revenues, but only because it is home to more scalable service industries and relies on the less lucrative but essential supplies and infrastructure from elsewhere.
What other natural resources are being "harvested" to enrich London so much?
Almost none of the UK's electricity supply originates within London, even if we take that to mean Greater London.
London also has no major sources for fuels.
Your whole thesis is laughable. You seem to be upset
Why would I be upset about anything? I only commented that London is able to generate disproportionate tax returns because it's service-based and relies on the rest of the UK for support. That's not a "thesis", it's a simple economic fact, and everything I've mentioned in support it of is readily verifiable for the price of a Google search.
As does rUK, with rWorld. As do most UK cities with rUK. Similarly with farming; London _and rUK_ can (and do) fulfil a significant quantity of their demand from abroad; it is possible to fulfil more.
I do not belittle farming, but trading for food does not equal "extracting rents" as the OP you were responding to requested, and nor does farming constitute a huge industry rUK is somehow being defrauded of the profits for.
Almost none of the UK's electricity supply originates within London
Energy production is not a natural resource. The only "rUK" resource we input is Gas, but it's not clear how much of rUK can claim to be losing out here. Most of rUK would not be the natural "owners" of this resource, and - again - it is traded on the open market, so can be fulfilled from elsewhere.
Why would I be upset
To rephrase/reconsider: you seem to somehow confuse trade with rent extraction, unless you simply did not read the OP's point of contention you were contributing to.
If this were to be true, rUK would be one of the world's worst perpetuators of this form of wrong, given its greater dependency on rWorld in exactly the manners you describe.
But even if you can give that up and you have enough supply to import all your food from elsewhere in the world, how are those imports going to get into London? There's limited spare capacity for freight ports on the Thames. Even if you could finish the London Gateway -- which isn't actually in London anyway, and you probably couldn't build anything like it further in -- and significantly expand its capacity from what is currently planned, you'd still need the warehousing and logistics to go with it, and then you'd need all the staff to build and maintain and operate all of this, and then the supporting infrastructure and services to go with those staff. That's an awful lot of new space and new relatively low-paid workers to fit into your hypothetical newly-independent London, just to make up for not having land-based supply of food and drink.
So again, as I have said repeatedly, London is only disproportionately strong economically because it is primarily based on high-margin service industries and relies on the rest of the UK to supply much of the low-margin infrastructure it needs for the basic operation of a city that size. You can argue over whether that beneficial financial arrangement is literally rent-seeking, but certainly the economic effect is similar: London currently benefits disproportionately from the UK's public spending and government-supported infrastructure, but that spending and infrastructure development is only possible because of what is also going on elsewhere in the UK.