My view on big cats in britain is pretty simple. While there may or may not be an actual established breeding population of any particular species, there are enough ignorant, misguided individuals that do keep them, then release them when they realise they can't cope, for whatever reason. And on top of that you get the not infrequent zoo/wildlife park etc escapees.
And again, let's not forget, that to all the people that go on about licencing etc, of dangerous pets/animals etc, murder is illegal in this country, heroin, crack cocain etc etc is illegal, but HALF THE COUNTRY is taking allso