just did a reccy of this route today with my flatmates. unfortunately i can't make it next weekend but i thought i'd give a bit of opinion/advice.
route feels mostly downhill with a couple of pretty long descents. particularly just before the M25 there's a steep long one.
None of the climbs were difficult on 70GI.
I wrote out directions before the ride, we didn't get lost. if anyone would like a copy i'd be more than happy to share.
Devil's Dyke is definitely worth doing, just a short detour up to the highest point.
All in all a great ride. Have fun next week!