Try travelcover.co.uk...we are both in our sixties and with pre existing medical conditions and got very good quotes from them...my annual policy is £59.00 and my partners for a 17 day cruise was £16.oo...another company we have used in the past is Insure and Go. You can get quotes on the internet for travelcover and if its OK can book it there and then.
Hope this help.
If you get no joy or it is more convenient try the insurance offered at The Post Office they have a fairly broad spectrum with their criteria...regards Id.
Both Saga and Age Concern will be worth contactin.
Try Help the Aged or Age Concer.