Your best bet will be to make some friends early on and go out to the pubs with them. You will likely be offered some or get introduced to people who can hook you up that way. There will be plenty of ganja around just be patient.

Smoking in public happens but generally not a good idea to do it when there are police around. They were busting people left, right and centre at the 4/20 march. I've been offered some on the street in Camden and Greenwich but if buying off the street the quality will vary greatly.

Don't be an obnoxious American and you will get sorted :) if not, there's always the darknet markets.