Over the past week I've been trying PAYG sims from O2/Giffgaff, EE and Three. I am moving from Vodafone. I live in Porthcawl South Wales and Park Gate just outside Southampton.
I've been testing coverage and data speeds at and around these two places and all along the route in between.
As always, any network may work for the places you personally go but my findings are:
Vodafone = crap
O2(Giffgaff)=crap
EE and 3 both great with 3 having better speed overall but EE having more 4G availability.
At the moment I'm thinking of staying PAYG with 3. The only drawback I can see there is lack of tethering. The pull of unlimited data is very strong. 300mins, 3000 txts and all you can eat data for £15 is great IMHO. I always buy devices outright so contract/device tie ins are no issue.