I was on a contract with O2 and had already unlocked my phone in anticipation of changing provider. I needed a PAC number to port my existing number to the new provider but when I rang to get one they convinced me to stay. They offered five times the allowances for two thirds of the price - a no brainer.
I use Giffgaff , as do some family members. What I like is it is a rolling monthly contract as I dislike long term contracts. I have unlimited phone minutes, 3gig downloads and unlimited texts for £10 a month. My wife has a £7.5 a month tariff for a bit less minutes. Giff Gaff uses the O2 network.