We're booked at Sherwood for this year - fri-mon before christmas. It's fab. I have 2YO, 5YO and three lovely teenage DSDs - all of them love it. Accommodation ranges in price depending on standard and size - best to check their website.
Last year santa visit was £10 but grotto was really lovely, plenty of time with santa, Elves were really kind and a decent gift which would be around the £5-£8 mark, so fair price all in, and I'm willing to pay it again this year. Add on the 5min pony ride to santa and the price gets a bit steep (but then 4YO loved it). Real reindeer in enclosure near grotto are free to visit, and all the toddler/young kid suff is all fairly inexpensive (Christmassy arts/crafts, treasure hunts etc), kids climbing wall was about £8 for half hour, and badminton about £8 for 45mins. We like walks in the woods and collect pine cones and spot squirrels, and go free swimming, the adventure playground and swing park every day.
Fireworks were free and fabulous, more like a lights display with lit-up hula hoopers and lasers as well as fireworks (well was last year) - wrap up well outside, or view it from inside Hucks with a hot chocolate (book a table WELL in advance).
We eat one meal out (usually pancake house) and self-cater rest of the meals (I bring a big frozen homemade chilli, curry, lasagne etc) to keep costs down.
To us it's fab (caters across the age ranges of all the kids) but I know its not everyones cuppa 
. We don't tend to take/hire bikes on the christmas break (we take/hire in spring/summer), instead we walk everywhere.
sorry, that was very long 
. can you tell we love it?! Sorry, Longleat is the only one I've not been to, but they mostly follow the same profile.