Can I fly a Radio Controlled plane anywhere I like in the UK (within reason)?

As I understand it you have to get permission from the landowner, while I am sure you would not intentionally cause 'nuisance' that would be for the person(s) whose land you were flying above to determine. Nuisance could just be engine noise but greater nuisance would be concern of potential harm or a fire if it crashed to persons, stock, wildlife, property, traffic etc.

Course David there are also plenty of targets like Terry Taliban for the Americans near where I live and they have my permission to fly over my house and garden anythime.

The Americans fly them all over Pakistan and Afghanistan and they make a nuisance of themselves all the time, blowing people up, assassinating their enemies.

I bet they haven't got a licence!