Actually, "1:1" Comets aren't that rare as far as I know. They certainly appear to be the most
commonly-seen WW2 British types at museums.
Like the US Pershing and Chaffee, it was a late-war type that continued in service for quite some time
after the war, rather than being scrapped soon after. Therefore when they were finally taken out of service
(I'm not sure of the date, but I think it was the 60s) quite a few ended up in private hands.
The "Cobbaton Combat Collection" museum in Devon, which I visited earlier this year, has two - a restored example
on display, and a very battered-looking one outside awaiting restoration.