O2 have recently announced that any user going abroad with their phone must disable iMessage in order to stop any data roaming charges. Turning data roaming off will not stop iMessage charges if you receive or send any iMessages whether it be to other users abroad or back to the UK. This may only apply to O2’s network so it’s always worth checking with your network to see what your data costs are in terms of iMessage.
You can disable iMessage in the iPhone’s settings menu (Settings -> Messages) to ensure you do not receive any data charges. Make sure you disable data roaming and mobile data too! (Settings -> General -> Network -> Data Roaming & Mobile Data [Off])