AFAIK you present the bike for a normal MOT test. You'll get advisory notices about the lights saying you can only use it on the road between sunrise and sunset and can't use it when visibility is poor.
The way I read the testers' manual is that to get the exemption from inspection on lights you can't have any at all (i.e. can't have just a brake light fitted for added safety when on the road as then you'd be expected to have all the others too and fail).
Horn needs to emit a continuous, uniform, constant note, not be harsh or grating, and be loud enough for other roads users to hear it.
From the small amount I have read on the matter, expect attention from the plod when using it on the road and have your documents with you.
They may not be familiar with the finer details of The Road Vehicles Lighting Regulations so the bit you're interested in is 'Regulation 4 (3) (a)':
"Nothing in these Regulations shall require any lamp or reflector to be fitted between sunrise and sunset to–
(a)a vehicle not fitted with any front or rear position lamp,"
