'Elveden Forest Village', Sufffolk.
Yes, I know, I know. But hey, we went a couple of years ago, holding our breath, and were very pleasantly surprised. We found that we were staying in a very comfortable 'lodge' in the woods, with a stream at the bottom of the patio and various woodland animals wandering around (eg. lots of ducks, swans, rabbits, deer, a heron and even a peacock!). The accomodation on the 400-acre site is set in woodland around a lake (photo above) and the 'village'. The idea is that you get around by bike (or stroll) down the no-cars pathways.
I have to admit that many parts of the (obviously, totally man-made) 'village' resemble and feel like an airport lounge, but the woodland, the lake, the outdoor activities and the tropical swimming pool are great. It may not be my first choice (and I'd only go because it's great with children; there's too many other places to explore) but it's not a bad UK option - I'll let you know how we got on. In the meantime, I came across this Guardian piece by someone who also changed her mind about Centre Parcs - I thinks she puts it well.