None of the Club sites in either the C&CC or CMC are adults only I'm afraid. There are a few of the CL's and CS's (private 5 van sites) that operate an adults only policy. For adults only sites go to the UKCS website for listings in that category.
We held an AGM rally there (another group I'm a member of) a couple of years ago and had about 30 units there, plenty of room on site, good toilet/shower block; useful to us was the fact that they had a large chalet/summer house type building we were able to use for our get togethers and AGM meeting. Would be happy to visit this site again. But...................If you are looking at June, we like several others are usually on the other side of the channel from late May till early July........so, I'm afraid wouldn't be able to join you.
Last time I did Harwich to Hoek van Holland I seem to remember that all the details were on the confirmation email and that is all I had with me; didn't even need to produce that!...rolled up to the check-in and without producing anything they greeted me by name (obviously using ANPR).