International roaming costs are always high by any network provider. Even though you activate any roaming plan, Still you would end up in paying huge bills. You can switch off data pack and start using wifi. But you can't expect for a wifi hotspot wherever you go.
The better thing you can do is unlock your mobile phone and use it with a local SIM from UK. You can unlock your phone safer and easier using unlock code .You can get the unlock code either from O2 network or from any trusted online vendor like
http://www.onlinegsmunlock.com/ Good luck.