I am with Doghound on this one, I think many people genuinly beleive they have seen a big cat, but perhaps they saw something else, Dog
Otter, Large domestic cat. Dog who knows.
The absence of hard evidence is the thing that ruins the show, If as is claimed these animals are the result of releases years ago then there must be a robust viable breeding population this would require a large area , Within this area there would be a requirement for several static populations to allow breeding pairs to take have kits , with the consequential increase in kill density. I can not beleive that a population such as this would remain undetected.
There are more than enough stray / and feral dogs and cats in this country to produce the animal kills that are reported.
I think it is plausible that in the past the odd animal has escaped or been released into the wild but breeding and large population no.