I think cinemas have night vision cameras installed so they can see if people are illegally filming the movie, and the same may well apply to them checking masks (someone may be asked to watch the screens and challenge (or potentially call the police) on anyone who isn't abiding.How likely is it that the cinemas will be checking - during the movie - whether people are still wearing a covering?
You can hardly run pre-film messaging, imploring people to switch off mobiles (or put them on silent), so as to not ruin other people’s enjoyment of the movie.... then switch on torches or put the house lights up, to check compliance on face coverings !!!
I was under the impression that at many TOC's the guard is simply in the back cab watching CCTV to check mask compliance and can and will alert the BTP to any concerns on route (although I may, and hope, I am wrong on that one)