englishal,
guessing that is a multi-trip travel policy. They usually have something like £2million coverage for liability, but the ones I've seen will exclude anything to do with flying unless you are a fare paying passenger. So if you pay the extra to remove the flying exclusion for medical purposes, do you also remove the exclusion for liability? If so, that would be an extremely cheap way of getting millions of dollars of liability coverage when flying in the US.