If it is one of the rear cats, it can be removed and replaced, at your peril with the MOT, with straight pipes. These cats are not monitored by an O2 sensor, but they do scrub the exhaust that last little bit to make it as clean as possible.

If the MOT knows it's supposed to be there, it might cause them to fail you. If they don't know about it, these engines run so clean that the exhaust sniffer should see a clean enough gas stream to pass you.