I have just been looking at new mot regulations from 7th april 2012, by the dft, to comply with european testing procedures,
it states, and i quote...speedometer, the car will fail if a speedometer is not fitted, is incomplete, INOPERATIVE, has a dial glass broken/missing or cannot be illuminated.
goes on to state other new rules which affect, electronic parking brake,electronic stability control,warning lights,lighting,electrical wiring and battery,trailer/caravan elec socket,tyres,restraints,seats,doors, towbars, exhaust and fuel system,
some of the above will not apply obviously to classic cars.