Non-event in physical terms IMO; slightly push/shouty confrontation with both “sides” likely very aware that they were being filmed.
I suspect that the representative of the owner/occupant of the building (if that’s who is wearing the hat – assume so, as they eventually let her in) probably has a right to take down stuff that’s been flyposted and the people doing the posting are trying to obstruct that. If the lass in the hat is a member of the public who disagrees with the content then she has no real right to pull it down and then it’s a simple disagreement over content.
Assume it’s not overtly nazi/denialist stuff or the police’d probably be there and they’d be hiding their faces while doing it. My guess is the posters and “installation art” are right wing and they’re outside a lefty charity/paper/party HQ.
My Summary: low-budget Tommy Robinson style publicity stunt outside The Guardian
Edit – wrote that while trying not to read the poster text; “linksextrem”, so I’m sticking with the above (though I guess it could be promoting extreme left rather than attacking it as I reckon).