Good luck Emmy, Beachy Head is too close to Stevenage for me. I am planning on doing it next year. I have done the LDWA South Downs 'marathon' 28+ miles and 4000ft ascent.
It's a beautiful place assuming there's no sea fog or high winds which can make the route entertaining and fairly challenging.
I'm hoping that it'll be good weather. A friend of mine that has done 10+ beachy's said that you can never be too sure about the weather so i'm thinking of taking my raidlight backpack with some spare clothes/etc