Switched to 3 from Vodafone as I didn’t like paying £5 (now £6) a day to use phone while abroad, plus never saw a 4g signal unless near London.
3 signal indoors is hit and miss though, and their customer services seems to be India based, but it’s £5 a month cheaper.
Had previously used a 3 PAYG sim for overseas, as part of their (3) feel at home service.