Ok so looking at June Saturday dates, it looks to me like 16 June will be the darkest evening, earliest and for longest:
June 9
Sunset 21:15
Moon is at 72% and sets at 10:45
June 16
Sunset 21:20
Moon is at 9.3% and sets at 18:32 (so before sunset making it nice and dark)
June 23
Sunset 21:22
Moon is at 15.6% and sets at 23:10
I'd like to suggest 16 June at Devil's Dyke, hoping that being a June date, we won't be subjected to wind chill of -20C again.
I also have free run of a local golf course which is quite well sheltered from light pollution and only 2 miles from Devil's Dyke, so we could go and startle the bunnies with bright things.