With help from Mark, Gavin (at policeboxes.co.uk) the Met List and one of the residents of the Britannia Village Residents Forum (a residents web board here in the fashionable end of East London) I'm now roughly 1/3rd of the way through transposing the Police box list onto a Google Map.
http://maps.google.com/maps/ms?msid=208425024888313928314.0004bc1271045907731b2&msa=0&ll=51.526688,-0.053558&spn=0.443426,1.352692
Its taken me a week to get this far..... I've played around with adding a few pictures and info to a couple of the boxes on there, and in addition to a couple of Mk 3's, theres also the red Blackwall Boxes highlighted too.
Very slow progress but rewarding none the less. I'm checking each and every streetview too, and theres little patterns starting to emerge... particular manhole covers, the positioning of GPO Junction boxes, gaps in walls / fences that work around recesses that are about the right size.
in fact, it got to the point where yesterday I was in Camberwell, and spotted a manhole/GPO box combination and thought "hello!" and today, noted that it was the location of Box L10... i should get out more!
Factoid: The furthest box out from the centre of the map appears to be Box J27, out on the Crooked Mile, near Waltham Abbey... miles out from the M25!
Also, theres a pic out there of a woman and a child sitting by a Box located by some park railings which a lot of people believe is a box in Peckham / Peckham Rye. I don't think it is. I believe it could Box L7, Queens Circus, Battersea. I could be wrong..... i probably am!

But then... theres a few more parks out there and a huge list still to work through, so that may change. But i really don't think its Peckham!
More updates as and when.
Happy Mapping!
Mark D