Bought a 48h Hop On Hop Off London ticket, and was very pleased with it. It contained an unlimited Thames river cruise also,which was a great benefit. My holiday in London was in early September and the weather wasn't so good, so the buses were quite empty - at least the open top parts ;) If you wanted, the tour guides offered a free disposable raincoat. In my opinion the "blue tour" which had pre-recorded audio was better. You had the choice to select the volume, mute the audio if you wanted and just enjoy the sights. The "red tour" had a live commentary tour guide. The volume in the bus was quite high, and you couldn't escape the tell-tale even if you wanted :) The guide in my bus was a very humorous elderly gentleman so no complaints there.
The bus routes covered the essential parts and sights in central London, and the timetable was very good. If you aren't in a hurry - and your hotel is located conveniently near the route - you can even use the Big Bus Tours buses for getting around in London. Keep in mind the traffic and traffic accidents, which can delay the bus's schedule.
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