BTW for a farily thorough list of Trackdays, we tend to browse
www.trackdays.co.uk then search for that date in google and book direct.
A good place to start for TDOs is
http://www.atdo.co.uk/
I've also been to Focussed Events - seriously packed circuit with lots of queing, ran okish on track but driving standards were poor and nothing was done about the most dangerous drivers. I complained about one BMW but the TDO pointed me to the marshalls rather than doing anything themselves (eventually he went home after the second time in the barriers).
MSV direct are also worth a look, the overall feeling is again a bit bucket shop with loads of cars. Their calendar this year has a lot more beginner and road car only days - if you fall under either of them it's worth looking at.
Lydden trackdays are worth a look, believe they only sell through their own website. Very different experience and I think are always sessions. That said you can overtake anywhere and on either side of the other cars. Not a day I'd do in an expensive car but great fun in the Saxo.