Two of us will arrive at the Heathrow airport at 12:30pm on May 16th, our connecting flight to Cape Town doesn't take off until 9:30pm on the same day. Exclude the arrival custom and check-in time for the next flight, we will have about 7 hours. Since it only takes 15 minutes to get to downtown London via the Heathrow express. We plan to store our carry-on luggage at the airport and then take the Heathrow express to downtown. What should we do to get ourselves a nice sight-seeing tour in 7 hours? Should we purchase the "Original London Tour" ticket, hop on and off or just purchase a day bus pass at the station and create my own route? Any advice? Thanks in advance!