Most of the UK has laws or bylaws at least against antisocial drinking e.g. if you're being a twat, violent, homeless, etc you will be asked to pour it out and leave, in incredibly rare cases I guess you might be fined but probably not.
Just having a beer in public at a picnic with friends is fine and is a national pastime.
Uh, citation needed?
Some small parks, cemeteries, kids playgrounds maybe
Every large park in London at least is full of people drinking
There's even a kids playground next to a pub in London fields where I often go drinking with other parents while the kids play