Headlight and other lighting requirements aren't covered in either the MOT (other than beam pattern & alignment and that they all work) or the Construction and Use Regulations, but are regulated by the Road Vehicles Lighting Regulations 1989.
http://www.legislation.gov.uk/uksi/1...chedule/4/made
If you look through the various schedules you'll see that there are very relaxed rules for anything originally built before around 1972 - 75, which would only be 'overruled' if an IVA test was required, in which case all bets are off.
As an example of what's possible, for the first three years I ran my Spyder ('68 Vitesse donor) with just a single, central brake light, which passed its MOT every time as it was within the regulations.
I only changed to a pair because people kept hooting me in traffic and shouting that two of my brake lights had failed.