If the aeroplane has British registration then CAA rules apply and any flight within the FAA definition but outside the CAA definition of aerobatics is permitted.
Aero's in FAA land is defined as > 30 degrees in pitch and >60 degrees in bank. Some "normal" aeroplanes, like the DA40 are certified for bank of up to 90 degrees, which is aerobatic according to the FARS....
You could do it safely in a DA40, but if you are worried about breaking the rules, would bust the FARs unless you wore a parachute - one of the requirements for aerobatics in the USA.