From what I have come across, even if it is an original fitment, you can remove it without risk of MOT failure, as long as your other two brake lights work.. To take it further, if you simply disconnect it and tell the tester that it is disconnected, he is obliged to believe you and still pass it.
The only failure is if it is in place, connected and doesn't work due to perhaps a wiring fault or blown bulb.
