Last year I used a Montane Lite-Speed H20 which is waterproof and has taped seams, very light weight and rolls up very small. Seemed to work a charm even in the driving sleet. Cost me £45 from SportsShoes, I think it may have been discontinued but there are still some around for bargain prices, beware though that it has to be the H20 model as the standard Lite-Speed is not waterproof and doesn't have taped seams.
I find running up and down hills uses my muscles quite differently to running on the flat. The uphills stress my glutes, the downhills my quads. If you are planning on a hilly race then hit the hills in training. I would probably mix it up between a few long reps and many short reps.
I don't know the route of the White Cliffs run, but it is likely none of the hills will be huge, but it will be constantly up/down/up/down and some of them will be steep which can make it difficult to get into a rhythm. When I did the Jurassic Coast a few years ago I used the steep hills as an excuse to walk.