I am an authorised examiner and on an MOT test, the exhaust is only checked for structural integrity, security and for severe leakage (a minor blow is an advisory). It will NOT be a failure in itself if the cat has been removed. If the car can come within the limits set on the emissions or smoke test, then it will pass, simple as that. If it fails the emission/smoke test on excessive output, then it will fail on that alone as the tester can not say whether it is the omission of the cat that has caused the high emissions/excess smoke, or another factor (say faulty engine temp sensor for instance). On my forum it has come up time and time again, members asking will my car fail its MOT if the cat isn't on. The definitive answer; If it can pass the emissions/smoke test, then NO it will not fail.