Is this law exist today for motorize bicycle UK?

i have 50cc engine fitted to my pedal bike. i been told that i can still pedal my bike perfectly legal. long spark plug taken out of the engine can not be propelled by the engine and become man power totally legal is that true or not!!!.Or is they anyother way around it. i don't see the point that electric is legal and engine is not.am only getting max speed of 25mph max i know electirc bike go beyond that speed depend how many battery u fit on your electric bike. consider electric bike weights 3 time more wth battery pack fitted to it.i alway offroad alot with mountain bike in woods but alway worry if get caught with engine fitted to my bike when pedal my bike on road get it offroad. even tho am not riding my bike with engine switch on.