I think most people say
1.EE
2.Vodafone
3. O2
4. Three
For the last couple of years I have tried all the major networks on a Sim Only deal and I found EE to be the best. I also live in London and there were certain places where I did not get good speeds/signal with Vodafone. However. I can't think of a place I have been so far with EE where I couldn't connect to the internet/load a webpage. I have to admit with EE, you do get what you pay for but they are very expensive.
I took out my 14PM on a 2 year contract with EE but I got 20% discount (work for the NHS) and with the free Apple Music for 2 years (which I would otherwise pay £9.99 for monthly) it made sense as I normally avoid phone contracts as they usually work out more expensive.
Avoid Three whatever you do, they are bad imo. If you are in a good 5G area you can get fantastic speeds but their 4G is terrible. You will have full bars on your iPhone but you can't even load a webpage.