Okay, so it’s not supposed to be totally free. At the end, you’re supposed to pay what you feel it was worth. Fortunately, that means guides work hard for your approval so these tours are highly rated, and there are 4 to 6 choices (from neighborhoods like the City, Piccadilly, or Brixton to single attractions such as the British Museum) every day, so giving up £10 or £15 in thanks should not be too difficult. It also does a few set-price tours such as a £25 afternoon pub crawl.