
It simply wouldn't work. The only vaguely possibly viable place would be the scottish highlands - but there simply isn't enough free area (or correct habitat) up there to support a sustainable population.
Back in 2003 I visited the highland wildlife park, and happened to reach the wolf enclosure in the middle of a talk - this was being covered by the keeper who was speaking. He suggested that Caledonian forest is the only really suitable habitat in this country, and there simply isn't enough of it - and it would take far, far too long to regrow some to the necessary quality (or amount) for a feasible reintroduction. He did give other reasons against an re-intro but it was 4 1/2 years ago, my memory fails :-P
Now the beavers on the other hand - that is possible. A couple of years back I visisted a reserve where a few beavers had been let loose in a carefully enclosed area. They had dug drainage channels and the like, but on the whole were a positive addition to the area. I imagine there would be potential problems with agriculture and the like, but there are areas where it could work.