I went to one near Reading last month. They had 35 cars, of all models. You register, say which cars you want to drive and then drive them, unaccompanied. For the popular models like SC430 it's best to go for them immediately otherwise even if you've put your name down, there's lots of waiting around when you get back from the previous drive.
There was a 12 mile circuit on the public roads, marked by yellow signs at the junctions - varied from small country lanes to dual carriageway. There was also an off road circuit for the RX-300, which was great fun, especially as it had rained solidly all the previous night.
I drove 4 cars, for about 20-30mins each: RX-300, LS430, GS430 Sport, IS300.
Highly recommended!
Peter