From The Guardian (UK) (Sept. 11, 2012):
Channel 4 has cited concerns over security as the reason for cancelling a planned screening at its headquarters this week of a documentary film questioning the origins of Islam.
Islam: The Untold Story, which claimed there was little written contemporary evidence about the origin of the religion, sparked more than 1,000 complaints to Channel 4 and the media regulator after it was broadcast two weeks ago.
Its presenter, the historian Tom Holland, was also the focus of substantial criticism, as well as abuse, on Twitter.
The channel said in a statement on Tuesday: “Having taken security advice we have reluctantly cancelled a planned screening of the programme, Islam: The Untold Story. We remain extremely proud of the film, which is still available to view on 4oD.” ...
[S]ources close to the channel said the screening had been cancelled after advice was taken from “relevant security authorities”....
The Daily Mail (UK) offers more: “Channel 4 has been forced to cancel a screening of the controversial documentary Islam: The Untold Story, after the presenter was threatened with physical violence.”
UPDATE: I at first failed to include the Daily Mail quote, which is a bit more specific on the nature of the security concerns; sorry about that.