Vehicles first used on or after 1 January 1971 must be fitted with two obligatory stop lamps.Additional stop lamps fitted and connected must be tested. Where extra lamps are fitted and there is doubt as to whether they are connected, the benefit of this doubt should be given to the presenter.
IIRC if a car is relatively new (post 95 etc) then it must be fitted with a high 3rd brake light which must work. .