In a letter to The Times (£), General al-Basheer, chief of staff of the supreme military council, the commanding body of the rebel Free Syrian Army (FSA), said the "majority" of foreign ISIS fighters were from the UK, with others from France, Germany and Belgium.

He said: "We, the Syrian people now experience beheadings, crucifixions, beatings, murders, outdated methods of treating women, an obsolete approach to governing society. Many who participate in these activities are British."
http://www.huffingtonpost.co.uk/2014/05/26/syria-britons-largest-group-isis_n_5392505.html