I thought that Saga can cover over 80s, provided you can answer 'no' to the medical condition questions?
If not, or the premium is just too high, ask yourself whether you really need such insurance for a 5-day trip into Europe when you are with close family anyway.
The Post Office offer insurance for over 80's, but you can't buy it in a Post Office, you have to ring up for a quote. The details are in the leaflets you can pick up in any Post Office.