Hi BB,
I wouldn't have thought that a Top mount can be an MOT failure. Two reasons why I think this is the case.
1. the MOT tester is not allowed to take anything off the car to inspect any part and the top mount is hidden in the turret, so its not likely they can fail you on something they cannot determine is a failed component.
2. even if the top mount is done for, it will only affect the geometry of the front suspension and make some sort of noise in certain conditions. The MOT tester does not test for geometry differences and they are not allowed to drive the car anywhere, so again they cannot determine a failed component that way either.
[EDIT] with the above two said, an easy way to check top mounts condition is to turn the steering lock to lock when stationary with the car running (with PAS assistance), if a top mount is faulty, it can ping or make a creaking sound as you go through the motion of going from lock to lock. The ping is usually a release of tension because the top mount is not free to rotate around the bearing as designed. It may be possible for an MOT tester to use this method to deduce a Top mount failure (given that the MOT tester cannot physically inspect the part) assuming that all other components can be and have been inspected to be OK. However I have never in my time seen an MOT tester do this test, so its hard to say if its a definite failure or not.
For example, on a previous car (back in the young and naughty days) I had a hole in the exhaust (which is an MOT failure), but at the time didn't have the cash to replace but needed an MOT, so the hole was wrapped up to cover it up for the MOT. The tester did mention there was a blow in the exhaust but was not able to confirm how bad because of my cheeky cover up (pun intended), so he passed the car. I don't condone doing this type of bodge of course, but at the time I had to keep making it to work. Goes to show they are there to check the safety of the car but they are also limited to what they are allowed to do to in carrying out their job.
I agree on the other subject, once there its best to replace it all and save labour costs in the long run.
Humour