It probably is a big cat, there are loads of them living wild in the UK. Lots of people got them as pets back in the early 's when Micheal Jackson had 1, same old story. . . . Cats grew, people couldn't cope so released them, they also brought in a new dangerous animals licence. Most people's enclosures were not to the correct standard.
These cats will thrive here, lots of deer and birds etc for food, lots of wooded areas for cover. The reason people don't see them again in the same place often is because their territories are huge and by the time they come back to the same spot. it eith