There are ALOT of rules for licence plates BUT being an MOT fail is a load of bollocks. In general they wont check. Ive had an arguement with VOSA when the police tryed fining me for the 3rd time this is after Ive changed it to the spec they asked andI said to them that if it was illegal then it should have been pulled up during the MOT which it wasnt. I personally think that you cant rely on all testers as SOME are lazy, or they are not informed in whats legal and whats not. Ive heard the same storie over and over. You have plates on your car which go through the MOT then you get told by the police that they're illagal.
Its the same with my reverse lights. They dont work and aint gonna work long story and I was told by a copper that it was a legal requirement...wrong! Its a legal requirment for makers to fit them but they dont even check them in the test!