adresses or map of north london post office letterboxes

"To find the nearest Post Office branch"

http://www.postoffice.co.uk/portal/po/finder?catId=7800129

i.e. your local post office should be able to tell you where these
letterboxes are located.

Can you expand on WHY you need them - there may be a better way of
getting the info you need.

I believe they may be on a database of street furniture, but not if
this only stores loval authority assets. Royal Mail might see this
info as commercially sensitive, especially with deregulation and
competition snapping at their fat, lazy, inefficient, incompetent
heels (sorry I almost slipped into one then!).

Bless them, until someone can replace them with a proper service.