Hi Bigman,
This should probably be in the Laning section rather than offloading section.
Most of the lanes in Beds and Cambs are closed in the winter, opening in April and closing in October. I haven't done many round here (I live in Burwell) - there are a few out on the Fen but I usually head over Thetford way.
Cambs, Essex and Norfolk have interactive maps on the web (you'll be able to find them via Google) and Trailwise has free but limited functionality. I would encourage you to get a full membership to TrailWise via GLASS (I'm a member) and that has a lot of information about which lanes can and can't be driven.
Not sure when I'm out next but if the group isn't too big I'll give you a shout - as we try an avoid big groups - usually no more than 5/5 vehicles max.
Where abouts are you based?