Depends what you want/phone is/signal area. I don't care about the signal charts that they all say etc, our property old, beams, nooks, crannies, wonky, lopsided, (that's the 1900's bit never mind the "slight rebuild" early 1980's.)
I have a doro flip on O2 pay monthly £5, it's enough for me. (Payg on Tesco/O2 is too expensive. I tried it this year and I was paying more. For me £5 is like the car, I might not need it, but it's there!) I've always been On the O2 network since it began as Cellnet, then O2 for several years, then Tesco for the last 10 years and now back to O2.
Saying that, I do have a smartphone (not for calls etc, too heavy and big for my bra,) that runs on 1p and Asda, both payg, but also means that all networks are accessible. It's paired with satnav.
I can't recommend 1p mobile enough! They are excellent! AND, unlike other customer services, they answer messages! (however, data in the house is dodgy! I can put my phone next to mums, she has full coverage, mine is struggling. But, across the fields perfect signal, which is why I joined them to start with, ability to watch a dog On a camera!) Also very helpful!
Asda are more expensive, but it's there if no other signal works.
Mum is on Talk mobile (vodafone) and the amount of data/calls etc for £7.95 is daft, she can't use it, but it's what she wants.
We have broadband but in powercuts, it's gone, so On goes data.