There was quite an interesting debate on another U.K. shooting website some months ago about the legal differences between a muzzle brake and a flash hider/flash suppressor. I have to admit that until I read that I hadn't really given the matter any thought but the firearms act actually mentions that items designed to reduce the muzzle flash require specific permission on a firearms certificate much the same as moderators while muzzle brakes don't require such permission.
You learn something new every day.