Usually I use streetmap.co.uk to get a quick O/S map of an area by postcode or street name, but today I’ve found multimap.com more useful (the interface is more graphical, the zoom in and out is better – it’s easier to use if you’re not sure where what you are looking for is in relation to everything else).
I found a map of all the counties of Great Britain and a postcode map of the same
This guide tells you what the different parts of the postcode mean. And a map and list of all the london postcodes (strictly areas and districts, but not sectors or street information) is here