Right. I've been on VJ since 2015, and I've never seen a denial for a UK applicant related to bachelor/bachelorette party, or a going away party. It really seems to be isolated to people from countries/cultures where there are elaborate engagement ceremonies that can resemble weddings, informal tribal weddings, etc. An English hen night is totally familiar to the London embassy (as it is just as common in the US), so I would proceed. Best wishes!
I suppose the social media question could be legit, but I think most UK party photos would be pretty clearly just parties, vs something ceremonial. YMMV.