me and kids did the touristy london thing in august. it was brilliant. had a half day bus trip which took us to westminster cathedral (brilliant) changing of the guard but not at buckhouse as they only do it on even days during the holidays or something. past downing street, thru trafalgar square. then dropped us at picadilly. we then used our travel card to jump on and off underground all day. we went to london dungeons which was good and then onto madame tussauds for our after 5 spot. it was ok but glad it was a cheap deal.
going to go again next summer and just wander about.
http://www.londontourist.org/beginners.html
http://www.timeout.com/london/features/2253.html