according to my neighbours, you can buy kittens, not do any of that and then just let them be feral around your neighbourhood

Ignoreing the not having to care for them. And legally cats must be chipped now. But that is a very very recent rule change.

This is very much a majority opinion in the UK that cats kept inside is cruelty to the cat. You opinion that all cats are required to be inside creatures is the rare one. More common in the younger generation. But not one backed up by evidence.

Bird deaths are the most common sighted evidence. But cats have been in the UK at least since the Romans first arrival. So 2000 years. And have been used as pest control on farms extensively since at least that period. Urbanisation may mean more cats. But the expansion of humanity and removal of habitate is the real issue.