Is this 144-hour TWOV really true?

I am flying to Beijing and stay for 4 nights and 5 days via Air China then fly to Tokyo, I am a British passport-holder. There is no official publication on the 144-hour TWOV that I can present to the counter when I check-in at Heathrow. I am worried that they will not allow me to board. I called the Chinese Embassy in London but they said they do not anything about it so I have to call the immigration dept in Beijing. I called but no one speaks English!