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Don't neglect the best way of getting round much of Central London - on foot! It's a surprisingly walkable city, even if the locals often ignore this fact. Just remember that the Tube map does not represent distances.

Any idea what you're planning on visiting North and West of London? Much of it is commuter belt, but there are some obscure attractions in there, as well as the nore obvious (Windsor, Cambridge et al). However, note that some of these can be as bad to park in as London itself, so you'd be better off by train, if you're not headed into the country (says someone who lives 45 miles north of London and hasn't owned a car for nearly a decade...)