Some festivals are urging the UK Gov to declare a policy on Vaccination Passports. Guardian article The article does suggest ticket sales or potential is high.

For events normally known for putting on dances I don't see an easy decision. Social distancing is difficult, even for partner dances like French dancing. And there will be reluctance to participate by the festival demographic.

Just yesterday I bumped into a ceilidhnaut who gave me broadsheet called "the Light (a truthpaper). As can be deduced, it was anti-vax, so there is resistance on several fronts. Though passports would filter that subset of the festivalgoers.