General rule for Mot is if it was fitted from the factory, it should be available at the time of Mot test. If you remove the rear belts, you run the risk of failing. Would you expect to pass if you removed the front seat belts? I have a 1960 series land rover which has no seat belts, but was made before they were mandatory so not required for mot. The Mot is a funny beast, you can fail for a cracked screen, but if you remove it and turn up with helmets and remove the wipers, no problem.