This is very cultural

In London on sunny days the park is 100% rammed with people sitting in circles on the grass drinking, from like noon to sunset

For sure. As others mentioned some locals have gone so far as to make drinking in public illegal.

Now in your example, suppose you’re a lonely stranger. Do you just nudge in on a circle with your beer and “Hi I’m Shawa” ?

Your answer may be yes, but in other cultures that’s going to get the police called, or maybe end in a stabbing. Which is why society is in the state it’s in

Cultures where people sit on the grass in extremely hostile drinking circles, ready to stab strangers?

Yeah, but people seem to call them gangbangers, drunks, meth addicts, and homeless camps. True or not, some cultures self-terroize.

The same people with who I drink in pubs in other times. Which happens quite frequently because it’s completely legal where I live. Also almost everybody does it.

So nothing extra compared to people who are drinking in pubs.