For the same reason you are not allowed to use Sports shells at a TC race, saloon shells in 8th Track F1, Indy cars in the F1 class - this class is for GT cars currently running in British and FIA GT classes.
The class is not for the chassis with any shell, it is for GT cars running chassis conforming to those rules. There are classes for Mk1 Escorts and all the other shells Kamtec offer, so if you want to run a shell there is probably a class for it. More shells will be available in the months to come for GT12, so hopefully there will be one that takes your fancy. HTH
