Part of a reply I had from GW regarding this :

Just so you know, the charges are aimed at larger bookings, as you suggested, and we will be clarifying the process for smaller groups, as we certainly want as many people as possible using our gaming hall to continue enjoying their Games Workshop hobby. I am also going to make it clear as to what size groups will count for the hire charges.
.which strikes me as good news really then, and reassuring for people/forums planning smaller, less formal get togethers.