Recently had a giff gaff sim for about six months, they use O2 network, no complaints, then I found out about Ovivo, and switched to them , much better deal in fact it's excellent, they use Vodaphone network. Have been with them for the past 2 months and have no complaints at all a much better deal and a person to speak to if you need help, which Giffgaff does not have and you have to rely on e mails also some of the answers you get on the forum are rubbish.
I have nothing to do with Ovio but give credit where it's due, in my eyes they are excellent and I recommend them, check out their deal it can't be beaten.
http://ovivomobile.com/