Up until the late 70s keeping 'exotic' pets (such as big cats) wasn't illegal, and plenty people did so. When the law was changed their owners had to either give them to some sort of insitution that could care for them (such as a zoo), or have them destroyed, in either event at the owner's own expense. There's no doubt that some of these owners chose to release their animals into the wild in more remote areas, rather than incurr the expense of moving the animal or having it put down. I don't suppose it's totally outside the realms of possibility that a few of these animals survived long enough to be spotted in places such as Bodmin Moor, often enough to create the 'big cat' myths still around today. That's not to say the animals are still alive today of course.