wes all the shit sims use the main networks anyways.
Talk mobile = O2
ovivo = Vodafone
can't say much for t-mobile as left them after they merged with orange and become ee as the signal turned shit for me, that much I want too 3!
I know what u mean GiffGaff and Tesco are o2 but a mate is on giffgaff and pa is on Tesco I'm on o2 and I always got signal when we're together and they never have!