I've been to Longleat and Woburn and one of the big draws is the swimming.
I liked that Longleat had a decent outdoor pool. But I like that Woburn had better kids pools inside. Overall I prefer Woburn because it's a bit smaller and easier to get a lodge within a good walking distance of the swimming. But those are the only two I've been to.
Out of all of the UK based Center Parcs, which is the best one and why? Which has the best waterslides? Which has the better pool facilities? Which isn't ridiculously hill?