I don't think the email is confirming entry, just more an update that you can enter on Tuesday.
I'm due to do the Beachy Head version in October, will start training soon...
I ran the Beachy Head in 2003, it nearly killed me. Hit "the wall" just before the Seven Sisters, around the 16m mark. I came in in 4hrs 21mins
I thought I'd be cool and went clubbing that night in the Honey Club, about 11pm my legs seized up and I had to get my missus to half-carry me up to the King's Road to get a taxi
Good luck and hope for a calm day