CAA definition of an aeroplane
I assume it's the same as everyone elses. That is, an aircraft (aero) held up by a plane (wing).
cf helicopter (rotorcraft), balloon (lighter-than-air), (others?)
In other words, there is no distinction between a "microlight" aeroplane and any other type of aeroplane. They are all aeroplanes, unless anyone knows different?
Russ