According to Christopher Morris, Barry, the Jihadist group leader, was based on a former BNP member who in an attempt to out-knowledge the Asian youths he regularly assaulted, studied the Qur'an and as a result "accidentally converted himself" and became a Muslim.
In the UK, the poster states "If you're reading this, find out what's wrong with you" in the small printed credits. As of October 2010, the poster is available in high resolution on the official UK website.
The last scenes set in London were actually filmed in Sheffield. The scene in "Kebabish" was filmed at Kebabish on the Wicker, in Sheffield. The other scenes were filmed on the Moor (a shopping area being renovated) and Campo Lane. Also, there are some shots where "For Sale" signs are visible and clearly show that they refer to Sheffield.
The trivia item below may give away important plot points.
During the credits, it is revealed that the bazooka Omar fired backwards in Pakistan killed Osama bin Laden at a training camp. In reality Osama bin Laden was killed by US special forces at a compound in Pakistan on May 2nd, 2011, five days shy of a year after the theatrical release Four Lions (May 7th, 2010).