I think I'm right in saying that if 3 are fitted to a vehicle,then,all 3 must be operational.If 2 are fitted then 2 must work and so on.
It is a valid defence if reported for defective lights to state in court that the bulb filament may have just blown before being reported and was operational at commencement of journey,providing of course that it was obviously not the case.i.e.brake light lense smashed and no bulb insideetc
retrocop: also if the police has spotted you with the light not working earlier that day etc or if captured on CCTV or AMPR cameras. Everyone always says "I'm sorry officer that bulb must of just blown". It doesn't work very well if the same person was seen 3 days ago by the same police officer with the same bulb blown. Police are rather nice when it comes down to broken lights - in my experience anyway.
I seem to remember that if fitted it must work for the MOT I was told this many years ago by a garage mechanic when I asked about the 2 spotlights I had fitted to the front of my car but had not got round to wiring yet.