GiffGaff - it's still relatively unknown but it's incredible! They use the O2 network so signal is never really a problem, and the prices are great:
SIM Only Deals and Plans | giffgaff.com
I have a recurring £10 goodybag so it's essentially the same as a normal contract, but not with a fixed length and it's a lot better value. You can bump yourself up to unlimited data as well for an extra £2 a month :)
If you do consider joining, would you mind doing so via this link?
https://giffgaff.com/orders/affiliate/tom982
I get £5 credit for referrals 
No worries if not though.
