If i went for an MOT and the tester failed to pick up on something as obvious as an illegal plate, i would be seriously concerned about their ability to pick up on other, potentially dangerous faults.
lol? The actual size of the plate wouldn't make it illegal would it? just the actual text size/font? It's not that easy to pick up on and it's daft to say that's as bad as missing something that actually makes the car a danger to drive.