A couple other things, GoinAllTheWay:
I found the
London Pass rather useful when I was there. I really enjoyed Windsor Castle, and the train from Paddington to Windsor is free with this. The London Eye was neat for about five minutes. Then I just wanted off. Get to a soccer game if you can and you haven't already. Way different experience compared to NA sports fans.
Also, the Heathrow Express train from Heathrow to Paddington station is far cheaper if you buy well in advance (90 or 120 days, iirc) than if you buy closer to your date. So if that (or the Gatwick Express, I presume) is one of your expected modes of travel, buy sooner.