There are big cats roaming Britain - without a single doubt.
If there are (bearing in mind not a single one has been captured) then there is a whole backstory of breeding populations and prey that doesn't fit what we know.
I'm happy to believe - especially in the late 70s - there may have been some big cats released into the UK by arsehole owners. However to survive and breed and continue to survive and breed for 50 years does start to stretch my credulity.
Of course there may be a steady release of one a year (for example), but that really raises more questions than answers.
All of which being said, there are people living in Africa and India who have never seen a leopard.