I've been to both Sherwood Forest and Elveden Forest. I think they are all fairly similar, however as Samsong7 says Longleat is hilly, which is why we haven't been there.
Some advice for you as your party are mixed ages, if there are some in the party who won't cycle and/or are not too good in walking, it is worth paying the extra to choose a central location villa as some of the other villas can be a long way from the main park area where the swimming pool, restaurants etc are based. Book activities as soon as you are able to as they quickly fill up. Watch out for the wildlife, squirrels and swans have been known to come into our villa when the doors are open!