Leave your details and they send you a window sticker to show you are taking part.
Though they may very well be boosting their data base of prospective customers another window sticker won't hurt and it gets classics out on the road.

http://www.heritage-quote.co.uk/drive-y ... k-week.cfm
You don't need to go through the riggmarole of having an insurance quote from them to enter. Address and e-mail and a few details of your car and journey.
