@tsmac - I think I know the man who are talking about. White guy, short to medium height, skinny with short blonde/ brown hair? Usually seen walking with a can of cider on Northcote Rd.
He mutters a lot to himself and once or twice I’ve seen him remonstrating with someone for something minor.
I haven’t seen him do anything serious (like throwing something), but I have lived in Islington so not too surprised by this kind of thing. Usually just avoid him, but do find him annoying.
I wouldn’t hesitate in reporting him if did anything untoward (like throw something at a window or bully/ verbally abuse someone). Best play it safe and report to #101.