I have taken and will continue to take a break from Disney but will respond to OP.
I was in the situation where I was looking to purchase insurance for my MIL living in Canada currently being treated for stage 4 non small cell lung cancer. She had been taking iressa for about 12 months before our planned trip when it started to show a mixed response and ultimately, disease progression was noted. That said, it was irrelevant to the insurance companies in Canada as to any coverage for this condition. I work as a lawyer and read the fine print extensively. We have a great system in Canada and I did not want to get stuck with some outrageous bill but it was my assessment that the policies available would only cover health issues unrelated to the cancer. In fact, even when policies purported to have coverage, confirmation calls with the underwriting group would indicate that anything directly or indirectly related to the cancer would be denied 100%.
Unless the disease has been stable, in our case, for 180 days, they pretty much said you are on your own. Further, after they spoke with underwriting after me making full disclosure about the drugs she was on etc., current status of treatment and next steps (she was to start on carboplatin and pemetrexed when we returned from FL), they advised that in her case, they would be of the opinion her cancer would never be stable a sad fact but true. She fortunately (or she would likely have been dead months ago) was EGFR+ for non operative non small cell lung cancer (adeno and some squamous) stage 4 with progression to lymphs in the chest cavity.
That said, if she was to have gotten hit by a truck or in a car accident, the policy would cover the same although my wife was of the view that even then, the insurance companies would find a way to deny the same.
Given the outrageous US health care costs, unless you can afford to self-insure (fortunately, my financial position is such that if we needed to pay $US50,000 to charter a plane back home we could), then proceed cautiously and dont really on the internet about what you can get or not get. Just my 2 cents, but posting on a board looking for advice from many Americans who are not going to have the ability to review the policy terms for such insurance issued by UK insurance companies and wont speak to the underwriting department with the companies that underwrite your insurance is not wise.
Just my 2 cents from my own research about policies in Canada, but unless the cancer is of the type that is curable, and when I mean that, treated and placed into remission for some time, and has been in remission for at least 180 days, the insurance companies here would deny any claims related to any issued directly or indirectly related to the cancer.
I wont be posting again or checking for responses or further questions. This may or may not be what you wanted to hear.
We went on the trip anyway and the Dr did not really sign off per se but understood that if something happened, she would have been flown home immediately versus seeking any assistance in the US. We only had 2.5 hour flight, however.