Well, Tim, given that the intention is to provide a separate race for the older cars away from the moderns, I'd hope the organisers would ensure that they only accept teams that will achieve that, otherwise it defeats the object of he exercise. Classes listed as per link below. I've noticed an error in the final para which refers to 6 hours but it is in fact 4 hours:-
http://www.750mc.co.uk/formulae/historicbirkett.htm