Aren't Evil Everywhere claiming 95% population coverage for 4G? Although that's a long way from 95% geographical coverage.
Sat on choo choo on way to that hellhole known as the smoke, and have had pretty much permanent data all the way. Mostly 4G. Dropped to edge in woods near Brill.
Everything bar Vodafone works well in my house

. EE is only one providing 4G, and then only upstairs.
When I was looking at ditching O2/NiffNaff I got some PAYG SIM cards and tried EE and 3 (didn't bother with Vodafone as doesn't work well round me, and had o2 anyway). My primary concern was universal voice coverage and near universal high speed data coverage. Which is how I came to my final decision.
3Gb is enough for my needs. I have 2 fibre lines at home and a dedicated adsl in the office, though I tend to use 4G at work as it's faster. If I'm starting to get tight on data, I use wifi in office and tolerate the sub 15Mbps. Also, obviously, I have BT Wifi which I can use when required.
You pays your money... ...but we are blessed with a competitive market, and thanks to EU, reasonable roaming when we need to visit euroland