According to the BBC "Sources say the cancellation came after China learned that British PM David Cameron planned to meet the Dalai Lama."
David Cameron and Nick Clegg met the Dalai Lama in London earlier this month. At the time, China's foreign ministry said the meeting had "seriously interfered with China's internal affairs".
Personally I think this kind of pathetic gesture politics only serves to make the Chinese government look like a childish and spiteful gang of bullies, as when they compared the Dalai Lama to a Nazi war criminal.
Those who want to see human rights and religious freedom in Tibet, China, or anywhere else in the world are not the enemy of the Chinese people, or of the nation of China. If the Chinese government chooses to see people as their enemies for meeting someone who has differing views to them but has always supported non-violence, that is their loss.