Mega Bus UK offers a 2 hour sightseeing tour on London for 1 GBP. This is a perfect option for anyone on transit to see all major attractions of London within 2 hours. My transit time in London Heathrow is 21 hours so i had enough time to take this tour. Visitor visa is needed to exit the airport for this trip.
This tour starts and ends on London Eye. To reach this place from Heathrow Terminal 5, i took London underground tube. There is no direct train. 1st train from Terminal 5 to Piccadilly Circus and 2nd from Piccadilly to Waterloo Underground Station. The tour start location is close to this underground station.
This tour can be booked online here: https://uk.megabus.com/products/london-bus-tour
When we pay online it will cost us 2GBP including payment processing charges. This cost varies according to the season. On peak season it costs more.
They have recorded commentary on the bus so we will be able to get to know about the attraction and historical details when the bus cross it.