I love London. I've spent a huge chunk of my life here during 2005-6. In my opinion, it's the easiest major city to get around in.
I started my journey with a direct BA flight from San Francisco to Heathrow.
If you pay a little extra (or use your BA miles) and upgrade to Economy Plus, the flight is amazing. The seats are comfortable, leg space is greater, a power port in every seat and the entertainment system has the latest releases (mine had "No Country for Old Men", "There Will be Blood" and other recent releases - compare to B-grade "comedies" I got stuck with on Continental flights).
In short - well worth the price. I hardly felt the 10 hours go by. Plus, where else on Earth will a stewardess call you "Love"?
About the only bad point about them is their less-than-stellar frequent flyer plan, with crazy tier demands, extensive blackouts and very easy to fall from grace with.
Anyway, I had a busy schedule and a major jet lag throughout my visit, but still managed to squeeze in some walkabouts. If you are pressed for time, and even if not, I recommend taking a London Walks walking tour. For 6 GBP, you get 2-3 hours with a knowledgeable and entertaining person. Usually s/he is an actor, or a comedian, but one was a lawyer (solicitor in British). They'll meet you at a Tube stop (Underground in English) and take you around a neighborhood or an area, pointing out things you'd usually miss.
I recommend the Spies' and Spycatchers' London and the various nightly pub walks (even if you don't drink). And if you have a Saturday/Sunday off try one of the Xplorer days.
I especially like the Holborn area. Nice book and music stores, theaters and great atmosphere.
I usually get off at the Tottenham Court Road stop and walk to Holborn.
My quaint recommendation would be an umbrella store (not kidding) on High Holborn, where I disappointed the proprietor by buying a 20 GBP ($42) collapsible model, rather than the more expensive and robust models he has demonstrated.
All in all, I lucked out and got a nice sunny week (which every Londoner would tell you is as rare as a 3 Pound coin). I'm missing it already.
I'll describe my short hop to Greenwich in the next post.