My other car's insurance includes track day the same as V's. 10% excess, cover as per on the road etc, so long as its a club event, properly marshalled and I tell them before hand.
However, as Colsoop said, one-off trackday cover does not normally cover 3rd parties cars, as it is their responsibility to get insurance, as they are chosing to engage in a risky past-time, fully aware of the risks.
ie if I hit someone else, then if I have insurance, my car gets fixed by my insurance co. If they have insurance their car gets fixed by their insurance co. If one of us doesn't have insurance, then they have to fund the repair themselves.