That sucks. It's going to be hard to get on the F11/10 now, especially as the V course looks to be finished. The last event a few weeks ago, the first 41 riders were sub 20!
Course records on H10/[1|2|8|17R|22] are all under 20:00, just do your 19 on one of those and you'll have no trouble getting a ride on the F11 to get your 18 :)
I got rejected from the Dunstable CC 20 on 7th July.
Apparently they had 250 entries so the cut off must have been a short 22.
It's a bit disappointing because I don't have that many free weekends in summer left.