Almost anywhere you can fly a small aeroplane - most of the UK except around major airports and airways.  You cannot fly over towns / cities, above the clouds or fly at night.

You must never fly closer than 500ft from any person, animal or structure (unless you have just taken off or are landing).  However this 500ft does not have to be a vertical distance, for example, legally you could fly along a sandy beach at 50ft as long as there were no dog walkers, sun bathers  etc within 500ft of the machine.