I'd see what the laws are in the UK. I know from managing other people that in the US you need to be very careful about anything medical related. As a manager I am not supposed to know about an employee's medical condition - if there is a reason someone needs to have a leave of absence for medical reasons - they send the material from the doctor to human resources and from their human resources lets me know what sort of accommodations I need to make. This is to protect the individual and their privacy and also to protect management from appearing to be biased due to their medical condition.
I would imagine the UK would have some laws around that as well - personally I do think it is awful and unprofessional - just simply gossiping about someone's medical issues and even worse to impact their work as a result.