hmm, that's one i've not encountered. You could always phone the CAA to get a definitive answer.
I don't see why you wouldn't be able to do a dual flight, as you need no medical for that, an examiner would then sign off the rating for a further two years. The licence itself is obviously not valid due to the lack of medical, but I see no reason why the rating could not be revalidated by experience.
Whatever your medical problems, have you explored the NPPL medical declaration?