BMFA says something about it being illegal to launch fireworks from a model plane.
a mate of mine taped a rocket onto the back of a model plane a few years ago, he got the engine running and the airplane ready to fly and i lit the rocket with a cigarette, the airplane took off just as the rocket lit, all went well until we realised that the payload on the firework rocket was a big banger.......the tail end of the airplane was blown clean off! one nice flat spin into the ground and the airplane was black bagged, funny is not the word for it, we we're wetting ourselves!

it's a great way of wasting unwanted model airplanes even if it is illegal.