I’ve got a feeling that the Bristol Ghostbusters are possibly the first British franchise, setting up in 2001, predating The UK Ghostbusters by at least 5 years. Fan clubs obviously existed before that, but fan clubs are rather different from full-on cosplay groups.
When the Bristol Ghostbusters founded, they created personas in-universe as opposed to now when people generally play either one of the OG characters or play themselves as a Ghostbuster. Bristol instead have characters like Samuel Thomas and Dr David Hugo (not their real names) and often still wear name patches with those characters on even today, 25 years later. Their gear at the time was mostly repainted Kenner stuff and packs made from foam/cardboard. Fibreglass shells very difficult to obtain in the UK back then. And of course we’re all spoilt for choice these days.