Castle Combe Race Circuit in wiltshire, just passed swindon on the M4 if heading from London. It's a wicked event with loads of nice minis plus the Mighty Mini racers take people out for laps in the race cars for charity. The sessions are mini only till the last couple of the day where you can take whatever on the track. I think it's about £25 for a 5 lap session which doesn't sound much but it seems more than 5 laps. I've taken my old mini a few times and I love the circuit but I've never taken a Gti round.
Nick