There are quite a few track day insurers about the place although they vary dramatically in price. A friend of mine recently insured his TT for a day at Cadwell - it cost around £150 from memory. Excess is generally 10% of value and from my experience third-party cover is unavailable.
It's worth having as as a part for your general policy if you can get it. I experienced a small bump in my Lotus last year and the repairs cost circa £13,000, all covered under my insurance. (No mechanical damage, just panels and paint!!)
I insure my Porsche with Mannings and that includes unlimited track days, the premium is perhaps £500 more than Admiral but I feel it represents exceptional value for money as I do 15 or so track days in that car during the year.
Try searching 'track day cover'
Cheers
