Skyline, I would be more than happy to pay £5 for parking, wash and shower I think that sounds fair, even if some chose not to use all of those facilities.
(I would be unlikely to use the shower as like karinofnine majority of time I ride trail centres alone)
Security is also a worry for me too, I've seen a number of threads now about people having bikes lifted from the campsite there, I'm always paranoid when I sit in the cafe, have to have the bike in eyesight.
*and I always thought you had to buy the tokens in the shop, which always has a pretty big queue when I look! Doh.