Reintroduction of animals into the British Isles?

The other species we have exterminated inside of Britain include Wild boar, wolves, a small amount of bears, and multiple bird and other mammal species. The main problem with re-introducing them is the fact the we have introduced non-native animals, which might affect them severely. The Grey squirrel for instance. Also, the animals no longer have the same amount of natural habitat, which leads to them entering cities, like you sometimes see bears in America stealing from rubbish bins. It would be difficult, therefore only possible for a few species.