Author: Bob Handford
One of the most popular sightseeing activities for first time visitors to London on their first day is to get an orientation of London on one of the Hop On, Hop Off Sightseeing Buses. The two dominant tour companies are the Original London Tour and Big Bus plus the Golden Tours service is slowly gaining ground.
Bob’s relevant blog and video of what to expect on the Original London Tour, (very similar to Big Bus in nearly all respects):
Original London Tour – What To Expect

Every year at some time during the winter period a promotion is run entitling everybody to two days for the price of a one day ticket. The only question is when the promotion will start and end, it changes every year. There is no special voucher or ticket or anything like that, instead of your standard ticket being valid for 24 hours, its valid for 48 hours.
In 2013/2014 the promotion has now been started by both the Original London Tour and Big Bus.
The offer is exactly the same for both companies. You buy a normal 24 hour ticket and as long as you travel by the end of February 2014 your ticket will be stamped for use over 48 hours.
The Golden Tours service has a 24 hour and 48 hour ticket as standard. These become 48 and 72 hour tickets respectively during the promotion period.

Original London Tour – Details
Big Bus Hop On Hop Off Tour Bus – Details
Panorana/Golden Tours Hop On Hop Off Tour Bus – Details