@ Mercedes Benz World in Brooklands, Surrey. 1 hour.
I went on one of these for my b'day last year and paid (or should i say my girlfriend paid) £170, but they have an offer on for £120 atm. (I think you have to book at specific times for this price, not sure) I'd take the day of work anyway!
Much better value than the R8 packages etc. at Silverstone as you only get a couple of laps for those.
The session is a hour long and you get a choice of AMG tuned Mercedes to drive (I had a dark blue SL55 convertible) you have an instructor with you and you do laps of their own track & use of the skid-pan. Really good fun & definiately worth a go whether your an experienced driver or not!
The museum is also worth a good look around aswell, full of brand new mercs to sit in and pretend you own lol. So all in all a great day out! (It was for me anyway!)
Not sure when the offer ends.http://www2.mercedes-benz.co.uk/content/unitedkingdom/mpc/mpc_unitedkingdom_website/en/home_mpc/MBWorldBrooklands/home/driving_experiences/drivingexperiences/driving_experiences/performance.0002.html