I did one of the HOHO buses in London and it was a colossal disappointment. Because of traffic and road construction (ALWAYS going on), the bus took strange routes to get around. I, personally, would not bother. You can get a much better idea of things by buying a good tourist map. If you insist on a HOHO bus tour, their maps are posted online - just be prepared for the bus to deviate from the route.
A much better way of going about things is to list your sightseeing stops and use the Transport For London journey planner web site to plan routes between activities. This will give you an idea how long and complicated the trips between sites are. It will also give you step by step instructions and list which tube lines/buses to take. This strategy should result in logically grouping your sights for ease transport. By the way, I highly recommend you purchase a data plan for your trip so you can use this website in real time so you can move around the city quickly and efficiently. I recommend purchasing a local pay-as-you-go SIM card for an unlocked smartphone...