This type of question has been asked countless times here, so I'll try to summarise the answers below:
Saga specialise in the field. Some AB members have found their prices reasonable but others have found them to be expensive:
https://www.saga.co.uk/insurance/travel-insurance/over-70
Age Co (formerly Age UK) Travel Insurance has been popular among ABers but, alas, is not currently providing cover (presumably due to Covid risks).
StaySure has quite a few fans on AB:
https://www.staysure.co.uk/over-70-travel-insurance/
Several people have found cover through Able2Travel. It's a brokerage that specialises in cover for customers with pre-existing medical condition but which also offers general policies to senior citizens:
https://www.able2travel.com/travel-insurance/senior-travel-insurance/
AllClear are similar to Able2Travel and also worth trying:
https://www.allcleartravel.co.uk/travel-insurance/seniors/
I've not been travelling much during the past few years (even pre-Covid) but, prior to that, I was getting my travel insurance at really low rates with Holidaysafe. (For example, four years ago, at the age of 64, I was paying just £16 for
annual European cover). They insure senior citizens, so I suggest checking out what they can offer you:
https://www.holidaysafe.co.uk/seniors-travel-insurance/over-65s/
Martin Lewis's Money Saving Expert website has advice and suggestions:
https://www.moneysavingexpert.com/insurance/over65s-travel-insurance/
Also remember that the big price comparison sites all search across large numbers of providers for the best quotes:
https://www.gocompare.com/travel-insurance/senior-travel-insurance/
https://www.moneysupermarket.com/travel-insurance/pensioners/
https://www.comparethemarket.com/travel-insurance/over-65/