This has been a thing since long before I lived in Britain, and continues to be a thing even now.
Big cats, Melanistic Leopards, Pumas or wildcats, supposedly from the 60's when they were kept as pet and then set free...
Amazing to think that a viable population of big cats could survive undetected in a such a highly urbanized area the size of Great Britain, roughly 1/3 the size of Alberta or Saskatchewan.
Links: https://www.discoverwildlife.com/news/new-dna-evidence-confirms-presence-of-big-cats-in-the-uk & https://www.discoverwildlife.com/animal-facts/mammals/big-cats-in-british-countryside
Maybe Bigfoot is real after all? I need to talk to Mrs. Notkenny Hicks...
And this gives hope for the Thylacine, "Tasmanian Tiger", frequently reported as sighted but no evidence has been forthcoming. That would be a success story.