When my husband had cancer with no prospect of a cure, many companies would not insure him at all, or wanted a sum that exceeded the cost of the holiday. Eventually we found that the insurance that comes with the Nationwide Flexplus current account would cover him. The account now costs £13 a month but you can recoup some of that in interest. You don't have to make it your main bank account. He had to pay an extra fee, but it was still much better value than anything we found elsewhere, and it was worldwide, annual travel cover.
Their main criterion seemed to be the number of treatments or drugs you're currently using, rather than your past medical history, and they didn't ask nearly as many intrusive questions as some other insurers.
It made a huge difference to his last couple of years, and allowed us several more lovely holidays together.