What sort of trackday cover are you after? Are you just looking for single event cover, as
Moris do that for about £70-80, depending upon venue, your experience, etc. They can also extend the policy to cover a number of events for even more money

.
If you are more serious about trackdays, you can get road insurance that covers you on track from
HIC (Hertfordshire Insurance Company). I took out a policy with them back in October, and it covers me fully comp for road use with a £150 excess, and trackdays with a £1000 excess

. I also opted for their agreed value scheme, so I don't have the hassle of having to haggle for a better price if I total my car

.
There are other companies around that you could try, but these two seem to cover the main options you might need. The alternative is not to have insurance, as it isn't required by the organisers. Obviously it is your risk, but if the track day is run properly you would be extremely unlucky to have an accident, as long as you know your car's limits

.
Phil