I think it is great that your father has so much get up and go! But a note of caution; and please dont think I'm being unkind. The reason why the insurance cover you mentioned is so expensive is because statistically, your father is a risk. ie they're not willing to put their money on his health for the duration of the trip. While I do admire the guy's spirit, I think it is a bad idea.
A trip through LHR is an awful experience even in the best of health. Any chance you could get them a beach break somewhere in the US...hell, get them to the Venetian or Paris in Las Vegas maybe?
