I'm a former Three user in central London. When I started it was good, then they advertised cheap unlimited data contracts which overloaded their system and they became close to unusable. You'd order an Uber, go down to meet it and be stuck because there was zero data. It wasn't a signal strength thing - it was a system overload thing.

I'm now on O2 which works kind of normally and also have a silent link esim which is a good backup. They cost like £8, never expire and let you use any UK network you choose if one isn't working. Or almost any network globally for that matter.