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.
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.