There are hundreds of maps for London, some for cars, some for cyclists, some for walkers. There are many different scales, formats, sizes; and I always come back to the Ordnance Survey map, for London, and for any other area I go for a few days holiday, or to live, or to give as a present.
These maps are wonderful. I often think that the British don't appreciate them enough, what with so many maps for sale, and GPS, and whatnot. But with an OS map and a compass (or the sun) you don't run out of batteries, you feel that you are able to work out where you are and how to get to places; as a bonus you get history, relief and a feel of place thrown in. Wonderful. I use them for car journeys (obviously, not for long-distance, but for detail), for cycling trips, and especially for walking - whether that is in Devon on longdistance footpaths, in Snowdonia for hillwalking, or in London for getting somewhere. these maps give you overview, and all the detail you need (except for streetnames).
The scale, 1:50,000 or 2cm per kilometre, is good enough for walking; I don't like the 1:25,000 as you walk off the edge of the map so quickly. For both route planning and the actual walk, this is The One! At last I have found a good waterproof jacket that has a breast pocket made for OS maps (Rohan hillwalker, yay) and it's perfection. A simple plastic bag around the map, folded to where you are, and even in driving rain you're OK - it's true, I have tested this in Snowdonia!
And last but not least, this is the right map for dreaming, too. I have a couple in a desk drawer at work; when I need a rest, I often take out a OS 1:50,000 and plan where I might go some day. Old OS maps (which you can pick up for a song in 2nd hand shops) are great for comparing past and present; but for a walk, or a dream, get the latest.
Thank you, Ordnance Survey, for some of the best maps in the World!