Yes they do day passes, I think it's about £2.50 a day, but the parking is free so it still works out cheap.
Also the shop that sells the passes doesn't open until about 9 or 9:30, a couple of times I've been for rides early and couldn't buy a pass because the shop wasn't open, so didn't bother. I've never actually seen anyone checked for a pass on the trails so it would be very easy to just never buy one, but someone must be putting in the work to maintain the trails and the car park so I don't have a problem with paying.
The small bit of Bracknell near where the trails are is actually pretty cool, especially if you have/work with kids, as there are the trails, a discovery centre, a Go Ape, and a leisure pool with uber-fast flumes:) Never been to the rest of it though so couldn't comment on that!