Giffgaff is a UK mobile phone network that operates Pay As You Go - there is no monthly contract, you top up your account with money online, and then you pay for calls / texts / data as you use it from your balance.
Giffgaff offer various bundles you can purchase that last a month, that for a fixed fee give you a set amount of call minutes, texts and data for free - more info is available on their website at www.giffgaff.com