Most UK policies have an exemption clause for driving on the Nurburgring, they mention it specifically as a route not covered under the policy. This is because it is actually classified as a one way toll road, not a racetrack. Without the exemption they would have to cover it if you have purchased a European cover policy.
However, even if you purchase European cover that does not specify an exemption for the Nurburgring, your insurance company will payout for any third party claims, but they will probably seek to recover any costs they incur from you in the event of an accident.
It is possible to buy insurance for your time on the Nurburgring, it is likely to cost several hundred pounds a day. Also currently policies for the Nurburgring only cover damage to your vehicle. All third party claims against you would still be your responsibility. One such company is MIS Motorsport. Cover starts at £150 per day for a car worth less than £10,000 with an excess of £1,500.