Years ago I had a chat with three young Muslim men as we waited in a Heathrow airport lounge to board a flight to Islamabad. I was going to Pakistan to report on the fallout from a devastating earthquake in Kashmir. They were going there to do what they vaguely described as “charitable work.” They dressed in white shalwar kameez, wore their beards in salafist style and spoke in south London accents.

I tried to steer the conversation to the earthquake. They wanted to talk about politics. Had I seen Michael Moore’s “Fahrenheit...