Age Concern offer travel insurance with no age limits:
http://www.ageconcern.org.uk/AgeConcern/travel_insurance.asp
Help The Aged own Intune Insurance:
http://www.helptheaged.org.uk/en-gb/OnlineShop/Insurance/Travel+insurance/
(Since Help the Aged has now actually merged with Age Concern, to form Age UK, you might find that both the above links produce the same quotes but, up until recently, they used to be entirely separate operations, so you might still find that one source is better than the other).
Some people seem to find Saga (who specialise in products for the over-50s) rather expensive for travel insurance, but it might be worth checking their prices:
http://www.saga.co.uk/insurance/travel-insurance/
The websites of UK charities for people with medical conditions often include links to Able2Travel. Able2Travel also offer travel insurance tailored to the needs of people over 64 years old:
http://www.able2travel.com/
Chris