@Tuppence
This is a problem I have also encountered and it does not seem to matter which company.
I can get travel insurance for lost luggage or theft but not in the event of cancelling because of a cancer recurrence. It may relate to the fact that I had to cancel a holiday trip due to my diagnosis just 8 weeks before travel.
I now think that I will "wing it". If I lost my luggage, I would go out and buy more, same for clothes. Airfares I would claim off my credit card insurance.
Insurance companies appear willing to insure for medical and medical evacuations as long as it is not cancer related. Funny thing is that they are happy to cover my asthma and that is likely to have me end up in hospital.
I now, would probably just tell the medicos that it would all have to wait until after the holiday. It was just my misfortune that my mammogram was in the June and we were going on holiday in the September. If the mammogram was in the October the cancer would have still been there - only I would not have known.
But now, I cannot bring myself to plan anything - just in case.