isit legal to roll a joint in a club or bar in londonWatch
Almost got thrown out of a Wu-Tang Clan gig at Camden Underworld in 2013 'cause I asked these 2 random girls for a puff on their joint and a security guard managed to somehow sneak up on me despite my vigilance. I apologised profusely, pointed out there was no smoking area and that the door staff wouldn't allow us to leave and re-enter, so what did they expect? Security searched me (fortunately this must've been the one occasion I went out in London without any weed of my own), found nothing, warned me that if caught again I'd be thrown out and permanently banned, and let me stay inside the venue.
Nowadays, I'd generally not risk it unless it was an event I wouldn't mind potentially missing and a venue I'm not bothered about returning to. What'll happen if caught probably depends on the discretion of the security staff and whether they can be bothered to make a scene with police or not. Might just eject you from the venue, might try to hold you somewhere until police arrive (although I believe they have no legal power to do so, average Joe public generally won't know that).
However you inevitably would be committing a crime by possessing the drugs to begin with.