Hi Steve, BT. I I save a fiver because I've got my landline and broadband with them. It's part of their "Halo" package, which doubles the data. It was originally 15Gb, then with each renewal and my threat to leave they up the allowance. First to 20, which the Halo doubles to 40, then last autumn to 30/60 Gb. To be honest, unless you're already with BT in some way you' won't get near this, sorry, but see below about iD.
Slightly off topic, it's always a battle to get thier broadband price down each year, but I can't get cable and don't want most of Sky's packaging, so I stick with BT. But back on the phone topic, have a look at iD, which is Carphone Warehouse's own mobile service - my OH has her mobile with them and gets 20gb for £8 a month. They use the 3 network. No 5G yet, but have a look at their SIM only deals on offer.
I've no connection with BT or Carphone Warehouse.