We go to Longleat annually, we love the place to unwind in a lovely setting.
It does not have to cost a fortune, choose your reservation dates carefully to save costs, their housing cost work on a basic supply/demand system...
Weekends cost more than midweek stays
Prices shoot up during kids holidays
Costs soar from July to September
Unless you have very specific requirements in the group, don't reserve a villa in a specific area, leave it completely open and save ~£50 IIRC.
Take as much food for eating-in as you can manage, the on-site supermarket prices are steep. There is a Lidl just a "stones throw" from Warminster train station, so even if you are travelling by train, you can do a shop there before getting a taxi from the rank immediately outside the train station.
Most sports at the Jardin des Sports are not too badly priced, but they cost more during popular times. They sting you for consumables like balls/shuttlecocks so take your own, take your own raquets if you have them. Snooker cues are free, but they are nothing better than pool cues, useless for spin shots.
Avoid ten pin bowling to save money, the prices are crazy!
The pool/slides are free, the fancy drier costs ~£1 (never used it).
The bird (owls/falcons/eagles) shows are great, very interactive, the weight of an adult Russian/Golden Eagle hybrid on your outstretched hand is quite shocking!
If you take your own bikes, take excellent locks for when locking up around campus, but keep them indoors (in the utility cupboard) overnight. There's a nice cat4 hill on the entrance/exit to the Longleat Estate over the road from CenterParcs, but better still, there are some amazing cat3 hills like Cheddar Gorge just ~30 miles to the west (I did a ~75 mile round trip from Warminster (~1000) to Cheddar to Longleat (~1700) last November on the first day at a gentle pace on my mountain bike, amazing ride).
The hire bikes are OK for around campus, a bit heavy, but cost ~£30 for the midweek stay per bike.
£500 for a 2-bed villa in October sounds about right, we are booked in to a 2-bed in May for £419, it can be as cheap as ~£350 to go around March but the weather is more erratic there in early spring.