This forum does not allow us to recommend particular insurance companies. It might be worth giving the support line a ring (they open at 10am tomorrow)
We were insured with Staysure, as OH suffering from dementia. Think it might depend how severe the dementia is, they always asked if he had deteriorated within the previous 6 months. We went to the Canaries.
When I travelled with my husband I used a couple of different companies. I think all companies will accept people who have a diagnosis as long as this is declared. Once it is declared you then have to answer a few questions. Of course the cost of the insurance goes up.
It cost me £1000 to insure us to go to USA last year, can’t remember the company but you can do a holiday insurance search for people with existing health conditions, just google it and various search engines come up.