There is no 3rd party liability on trackdays, that is basically what the indemnity is all about, accepting the risk of being on track.
Motorsport insurance works on this principal, you are insuring your car against various incidents. You can insure against just about anything including mechanical damage, it is just a case of how much you want to spend.
However, as is said here the value of trying to insure a car with a value of £450, is just not worth the agro. The excess will be more than the value of the car and even if you have a car in stunning condition its resale book value is peanuts these days.
You can insure for a fixed cost, especially if you have built the car and there a labour content involved, however you still have to hit these cars quite hard to cause enough damage to get past the excess point. A dented wing, broken light and bent steering rod can be fixed for a couple of hundred pounds or much less if you know where to look.