London Sight seeing in a day

Besides working in London for more then year as consultant, I got opportunity to explore the local sight seeing attractions thrice in same period. Immediately the next day after joining Axon, I was on bench for a day and was waiting to be assigned to project in couple of days. The recruitment team had asked me to stay in London. So rather then sitting at home, I decided to roam around central London. I took South West train from Feltham to Waterloo and reached Waterloo central station. I entered travel information office, immediately after getting out of the train platform. I was compelled to buy day pass for hop-off bus in London. I was new to London and was unaware that the zone 1 to 6 day travel card could take me to different destinations in London L and save my 25 GBP for Bus pass.
The second time I toured London attraction with Abhis parents and third time with my parents.
Over period of time, I have listed down an itinerary to cover major attractions in London central in day with cheapest travel and entrance ticket options.