If the car was registered before 31st December 1992, your car doesnt have to pass stricter emissions tests as part of the MOT. The car will have its emissions tested, but the CO2 threshold is higher. Most modern cars that have no catalyst fitted would pass this with ease.
For cars registered after 31st December 1992, the emissions test is harsher and most cars need a cat to pass it. But that does not mean you must have a catalyst fitted. In practice, some cars would even pass the test without a cat. It is not a legal requirement to have a catalyst fitted, although it is the law that your car pass an emissions test at any time.
Basically, you need a cat to get legal emissions - but you cant be done for not having a cat in itself. If a Police officer were to pull you over for not having a catalyst, he could not act unless it was proven that the car failed the emissions test.
I always go sans-cat as theyre expensive to replace and soak up a bit of performance. I just re-fit it at MOT time.
