It's quite possible to fail and MOT and still be roadworthy, they are two different tests.
I had this very problem years ago, and was told by Jean Paul, of Norwich MOT inspectorate, when he later examined my car, as I reported it, that the existing mot cert can still be driven on perfectly legally, I thne asked is my car dangerous to the extent that the plod can do anything, he said "no, it's safe to use, but won't pass an mot".
The problem on my car is it should not have passed on many things including corrosion, so was retested locally and he called the selling garage to view the car, actually gave him an option, to come over(he said he was busy) or lose his license, turns out that is what happened anyway.
I too was intrigued by the position of legally being able to drive on a cert of a test that it should not have passed.
tabetha