Prepare to be bored silly!
The rules And at the top of page 6, there is this:
(5) In the case of a motor cycle or a motor tricycle which does not have a body of a type
which is characteristic of the body of a four-wheeled vehicle—
(a) a registration plate must not be fixed on the front of a vehicle if it was first registered
on or after 1st September 2001,
(b) a plate need not be fixed on the front of the vehicle if it was first registered before 1st
September 2001.So the vital point is, what type is the body? Car type? If you argue it isn't, you are are arguing for one headlight only and a perpetuation of the idiot driving rules I keep harping on about. All for the sake of a numberplate.
Headlight requirements come down to type of body, I believe "car type" and "motorcycle type" are both referred to on this matter in the Construction and Use reglations 
