Competition car insurance and MORIS.co.uk do one day cover, expect to pay 10% of what you value the car at with an eye-watering excess. I've used these for European events in the past, the rates come down as they get to know you - a bit like a no claims bonus.
A few places will do you road and track policies, either on a 'x amount of days included' or £xx per track day' basis. Greenlight do mine and have done for years.