In the UK you have to remember that the MOT regulations are totally separate from the actual law / Regulations regarding Lighting and Construction and Use. Just because something is not required / tested under the MOT regulations does not mean it is not required by law. I.E the MOT does not check the operation of the speedometer, but one is required by law.
However, having said that, with regard to lighting it is my understanding that based on section 4 of the 1989 Road Vehicle Lighting regulations which deals with General Exemptions:http://www.legislation.gov.uk/uksi/1989 ... ion/4/made"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,"
This seems to say that a brake light is not required if these conditions are met. There is an interesting forum entry here regarding this http://www.trf.org.uk/forum/viewtopic.p ... 0&start=30
I do wonder what an Insurance companies position would be concerning covering a motorcycle which would originally have been fitted with lights and a brake light.
I suppose when you insure such a machine you would have to state that there were no lights / brake light fitted as this would count as a modification from original, which is one of the questions they ask.