FatAss - I can think of a few options:
Sherwood Forest / Clumber Park, I am happy to put a loop together to suit the distance agreed. Might feel a bit like the Dukeries event though (sorry Rog!).
Peak District, Again am happy to put together a loop however the terrain / ascent and possibly weather at Xmas may take some of the social aspect way with runners different paces becoming a bigger issue.
Rutland Water - There is a decent trail marathon run around the edge of the resevoir we could duplicate this, there are some lovely hotels on the shore we could perhaps get a deal with for the social evening.