If I was going to work with somebody on this there are two things I should throw out there right now...
1) I've got a lot on my plate right now with the two maps in production and a third that I'll be posting tonight. So if I'm going to do the graphics, I'd like somebody else to try and walk it through the Draft stage and work out where the territories, regions, and borders should be, bonuses, etc. That crap takes weeks that I don't have, but once most of that stuff is settled I can take over posting updates. This worked pretty well with the Scotland map.
2) As you may know if you are familiar with my past work, I like to have a historical setting to work in. We could do a contemporary map of London, but it's more fun to go back a bit in time and try to capture a particular era. I think this would be the right way to go with a London map, because if you make a contemporary map of London the city is so damned big you start wondering where to draw the line - somebody's street will not make the cut and be angry! And if you include everything out to and past all the airports you guys keep building, the heart of the city gets lost. Plus, who really wants that big ferris wheel on the map?
Keep the discussion going in the thread, and maybe this can turn into something.