I know it may seem to be a silly question but as I have a bus pass it can only get worse.

On the settings of the iPhone, General, Data Roaming should this be on or off while in the UK?
I have had it in the off position and it does not seem to stop any data getting through as far as I know, so is it needed and does it use more power when on??
When in say over in France I would make sure it is in the off position along with the Cellular data switch.