I'm doing it again this year - leaving early because we're riding home again
but look out for the RetroBike top
I think your kit list is fine. Weather is looking ok at the moment but you never know. Last year I managed to get away with tubes, levers, multi-tool, pump, phone, long-sleeve base layer, windproof, 'phone and cash all in sandwich bags in the shirt pockets. Plenty of places to top up with water on the way, a sandwich on the sea-front and off home again. I think we managed to sneak in two breakfasts and two lunches, followed by a huge afternoon tea when we got back.
My friend's tyre exploded about 2/3 of the way. The two-inch slit it caused made me think that was our day done, but luck was on our side as we were about 1/2 mile from a bike shop in Haywards Heath that opened half an hour later! A quick visit and we were off again.
All in all, a very busy day, but very well organised given the numbers. Arrive at Clapham early.