I would think that something like a transponder failure during a flight could be dealt with by sensible collaboration between the controller you are talking to and yourself.
I have been teaching and examining for quite a few years now and don't recall flights due to sick student being aborted as a common occurance. However again as you are in 2 way contact with the controller why do you think they would not be happy to let you abort the flight and return to base?
I don't see anythin the restrictions that preclude the aborting of a flight and return to a base. Again if you can show me something that supports your assumption then I would be glad to read and digest it.
I think there is a lot of assuming and doomsaying going on based on what i think is incorrect interpretation of the restrictions. A very typical British thing to create problems where non exist?