Reviving a two-year old thread? I don't live exactly in the triangle, but I noticed, pretty much throughout London, that:
outdoors, the signal of EE and Vodafone is roughly the same
indoors, Vodafone is much better. That's why I remain with Vodafone even though Three would be cheaper. I think it's because Vodafone uses a different frequency that penetrates buildings better.
Three is very much hit and miss
Don't know about O2. Wifi calling woudn't work with my phone so I'm not interested
I agree with the others that you should buy a bunch of pay as you go sims and test for yourself.
Two more factors to bear in mind:
wifi calling: not every unlocked phone works with wifi calling. You should check with the network, especially if you didn't buy your phone from them. Typically wifi calling doesn't work with pay as you go, so you'd need to splash out on a 30-day sim if you want to test that. And I'd recommend you test it before committing to a 12-month contract.
femtocells, ie the signal boosters that connect to your wifi. If wifi calling works properly, you don't need this. Vodafone charges for them. Other networks sometimes give them out for free if you live in an area with a poor signal - you should ask. I had an EE signal booster in the past, and the problem was that sometimes the phone would prefer to connect to a poor 4G signal rather than to the stronger 3G signal of the booster.