Height is difficult to judge from the ground. However, the legalities are not to fly under a thousand foot over a gathering of (I think) a thousand people or more or a built up area. What the defintion of a built up area is I'm not sure. You certainly need to be able to glide clear of any built up area in case of a donk failure, so say if I'm flying over Manchester I would want to be at substantially more than a thousand feet.
If he/she was at a thousand feet and performed a barrel roll which didn't go under a thousand feet and they could glide clear and he had done a HASSL check before hand then really he/she had done nothing illegal.
Having said that I probably wouldn't do a barrel roll at a thousand feet over the outskirts of Manchester.