Can only really speak for London but after many years I am leaving 3. Their signal is just shocking; large parts of where I live, most major transport hubs in SW London, on the way to work, on the way back home, random parts of central London, inside most buildings, bad weather and God knows what else - all of these have none-crap signal. My signal suddenly drops and then pops back up again, I often get fake signal (bars show, nothing there) and streaming the most basic YouTube content can be a struggle. Using Maps in London? That's about a 50/50 chance of it working properly.
This is across multiple devices and sims. I rang them up to leave the other day and with no surprise, they obviously tried to claim they could sort it out. I told them at the start of the call, I bet they will agree with me it's clearly an issue on their end rather than a single fault but they still tried. The first postcode I gave them......."looks like there may be issues in that area, will be fixed shortly".....fancy that! The next several postcodes I supplied turned out to be "maximum capacity" and the call was over fairly quickly. I hope EE are better.