How much are the hop on hop off buses in London?

When I took one it was £12 for a ticket good for 24 hours, but that was 7 years ago so it's probably a bit more than that now.

BigBus is £20 for 48 hours.
OriginalTour is £16 for 24 hours.
TfL Bus Pass is £13.50 for 168 hours.

Or you can buy a one-day travelcard for normal buses for £3.50.

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Or you can buy a one-day travelcard for normal buses for £3.50.
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which are also "hop on hop off" in nature.

But since you may not have a clue what you are seeing from the regular city bus - for a first timer the extra £8 or £9 for the tour bus is probably worth it.

Plus the tour will take you past all of the major sites w/o having to figure out the different bus routes.

Don't get me wrong - I take buses (and the tube) everywhere in London. But for sightseeing and being driven past all of the "biggies" and hearing a bit about them, the hop-on-hop-off tour buses are easier and more efficient.