Having ridden it once every two weeks the last few months, I can say that it will certainly be a "big ring" climb for the pros. You can do the whole thing without feeling the need to stand up. It is a piece of cake (much like the delicious and overpriced ones at the cafe on the top) compared to something like Ditchling Beacon or that more serious climb in Surrey (can't remember the name).
The new surface is the best I've ever ridden on in the UK, which is good because it used to have speed bumps.
All in all, an easy climb for a pro. Nine of them surely pile up, but with 40k of flat left, there's not much that can stand in the way of a sort-of-reduced sprint. Cavendish will have a small team, but he will have top guys, and he is ultra-motivated, losing weight and all. Sagan will have no problem with the hill. Germany will probably want to help pull any breaks back, in an attempt to get silver or bronze. Maybe Australia as well?
I think only the Spartacus of 2010 could attack on Box Hill and make it all the way to London. I wouldn't give it more than a 10 % chance, because his Roubaix long attack was helped by the cobblestones, which Surrey sadly lacks.