Easiest thing (if you've not already done it) is to find a local CBT centre and spend a day with them. It's not riding on track but it's cheap, probably £70-80 for the day and will give you a good idea of what it's all about.
If you want more serious then you can turn up and ride at Donington as part of the Ron Haslam school (
http://www.haslamraceschool.com/ and click on "On Track" ).
I believe that there is also a riding school at Mallory Park, a couple of friends have recently done their direct access there.