I wanted to agree with our very knowledgable Officer Rushjob on this one, BUT....
While Schedule 1, Table II says that a motorcycle must have a headlight, Part 1 section 4(3)(a) says that nothing in the regulation apply (i.e. Table II) if the vehicle doesn't have a position light, and it don't say anything about original equipment either. So take off your position light and you don't need to have a headlight!
So I would say not having a headlight doesn't contravene the The Road Vehicles Lighting Regulations 1989 and therefor is perfectly ok!
But I'm no legal expert, so does anyone know of a test case??