[afarrell]

A big part of the problem is that when you just walk up to someone they don't really know what your intentions are. My wife is actually currently working on this problem though. She is an event planner and has started running meetups specifically for people who are looking to meet new people. It has worked pretty well in the US, but we're actually going to be moving to London (assuming my visa comes through...).

[asksol]

People need more venues to meet new people, but by greeting I meant more like saying "evening!" when you pass your next door neighbor in the hallway. I don't think there's an expectation to chat in this case. If I wanted to chat I'd start with "excuse me," and probably walk up first as you say.

A greeting can be made from across the street, and sometimes just lifting your hat in response is enough (I don't have one, but I have seen people do this).

[afarrell]

Yea, that is in fact one reason that I wear a hat.

[dvanduzer]

The idea is to make yourself visible as someone who is friendly, so that people approach you. (Granted, this might not be something you want to start out with as a total newcomer to a neighborhood like London.)