Well, there could, and probably, 'easily' even, be more than a couple of species of big cats in britain.
Thriving, breeding population of a 'thousand' leopards, maybe not? But there could easily be more than a few breeding pairs, of whatever species of big cat in britain, easily.
Britain, away from the cities, has HUGE areas of countryside that someone could get lost in for days at a time, before seeing another person.
A wild cat of any size, even feral moggies, WON'T be seen if they can help it.