Is it legal peddle my bike on road UK?

i wanna know this right i have 50cc engine fitted to my peddle bike. somethink i put on for a daft laugh. am i still legal ride my peddle bike on the road with the engine not switch on.i can still peddle my bike with out the engine switch on. puzzle me i seen alot of country ride these motorize peddle bike quite legal. in UK it different nice if a police officer answer this or any 1 that know about the law. i dont wanna get into trouble just becouse i built somethink for fun. it would b my first time