Nina*
A very thorough tour
The Big Bus Tour Company has branches in major cities like Philadelphia, Shanghai and Abu Dhabi. Here in London, it offers a sightseeing tour. Rates may seem very high at first (GBP 26 for adults, 10 for children) but all things considered, the trip was worth it. We saw all the famous places in the city without the potential danger of getting lost, and the stops are very frequent (more than 50 throughout the city), so you can go back to them at your leisure. The audio commentaries are obviously very rich and give a good overview of the history of the city, even outside the usual tourist routes.
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