Richard,
SGS, and MOT are two entirely different things. The SGS inspection is to verify that the vehicle is what you claim it is. That is to say if you say that it is a BSA A10 then it must conform to that description, in other words does the vehicle match the paper work. If it does it will be issued with an age related number plate. If not then it will be classed as a special and it will be issued with a Q plate which identifies it as not being a vehicle that was built by a commercial manufacturer. The MOT is to check a vehicles general roadworthiness. Brakes, lights, steering, emission compliance, and structural integrity. Pre 1960 vehicles have been exempted from the MOT because the assumption is that older vehicles tend to owned by enthusiast and are therefore likely to be well maintained. Obviously, there is a bit more to it than that, but that is the basics of what it is all about.
John