35miles? I very much doubt it 🙂
Allow 15 minutes of faffing per rider that turns up (so two riders = 30 mins, 20 riders = 5 hours), then add that to the maximum ride time for the slowest rider and you'll find you have a short day. I've done many group rides (and photoshoots) where we've only ridden eight miles all day – it's just the Law of Group Inertia…
I'm bringing lights…
In past years I've always parked at Holford, then ridden over to the Bicknoller Inn for lunch (because you have no choice but to ride back up and over…) – I think cake and hipflasks might be wiser than trying to get lunch for 20 people. Any idea on a meeting place and vague time yet?