What he said. The MOT tests up until 2012 were purely on emmisions, so you could ditch your cat and take a (slim) chance that it would still pass. However after 2012 they change the rules that if your car came with a cat, it has to have a cat in order to pass the MOT.
In reality im not sure how MOT inspectors check this, as from the outside how do they know whether a cat has anything inside it.
I know people who "decat" their car by smashing the core out of their OEM cats, they are also talking about making it an MOT failure to have the ECU modified, however as you say above, for your average MOT tester, its going to be impossible to check!