There were 20 - 24 manufactures there last year with 4 or 5 (in Audi's case about 8 R8's) of there current cars.
It's been named the moving motor show, so it is a sales exercise of sorts.
You can't book in advance of Thursday, however you will need to book a time slot on the day so get there early.
The expensive cars are invitation only, so i doubt you will get into a Ferrari, Lotus or R8, but may be able to go in a camaro.
Off the top of my head last year there was Alfa, Arbarth, Chrysler, Ford, Peugeot, Citroen, Mercades, Bentley, Ferrari, Audi, Lotus, Honda, MG, BMW etc.
My membership card was in my welcome pack that i was given when i took delivery of my Giulietta 3 months ago.
If you are a member and have misplaced your card, phone the helpline number on the cloverleaf site and they will make a note of you so when you get on the alfa stand you should be able to get in. (i was told this on monday when they phoned me).
Last year there was an accident so the layout may have changed a little.
Fingers crossed it's a good & sunny day, and this year will be accident free