yeah should be fine, the engine management light will be telling you there is a fault, your car will have a lambda probe before and after the cat.
if the cat has failed - the rattling - the second probe will detect that it is not doing its job properly and will throw a code.
the second probe will not affect the mixture or the running of the car, it is there to check the cat is working correctly.
the cat will need to be changed, depending on how bad it is the core could move and partially block the exhaust, or it may just shake its self to pieces.
you will need to get it changed before an MOT as it should not pass!