if it states members only then no,but some sites do allow non-members and charge xtra,also you can join when you arrive,best to call them 1st and not just turn up.
If you check on the UKCS listing for a particular site, you should find if it's members only (as here) - if it's open to non-members this should be mentioned on the site's own website.
Don't forget to leave a review of the campsites you have visited this year or last!