I'm on GiffGaff with a Nexus 5 I got for £200 when they were selling them off cheap over a year ago. Been really good and saved me a lot of money compared to a contract.
But I also have a Three Sim which I use in the US and really do make a huge saving on data roaming (last trip I used data that would have cost over £6k on GiffGaff and only cost £20). I would happily go with Three all the time IF there was any signal in my house...
If you travel outside the EU then check the countries Three covers within the plan, if you go outside those areas too then you really need an unlocked phone so you can use a local SIM.
I need a new phone soon too and am looking for reputable places to buy unlocked UK phones too if anyone has any suggestions?
G.