As already said; depends about your location where you film it/ show it. For example; I wouldn't show even Disney cartoon in Saudi-Arabia. Active selling/ showing in US is borderline case, due to fucked up legal system. If you want to have some legal insurance that covers USA it fucking costs, if someone is even willing to give that. Not all domestic insurance sellers even want into that game. I don't know about UK, maybe there are more willing sellers.
In generally, if you film in private premises, you will need permission from the owner/ whoever has the right to use that premise. Porn or not porn.
Porn filmed in public place is another thing. Around here you can film in public place in generally. If someone is just on the background I think he has nothing saying in this. Unless you make him part of the scene somehow. Although I don't know the details regarding porn, as there is no such details at least around here, you have to apply other non-specific rules.
__________________
|