The Independent, March 2014

"The plea from the (ISIS)rebel fighter comes as UK authorities are becoming increasingly concerned at the number of British citizens travelling to Syria to join extremist groups in the three year old civil war. British and other non-Syrians travelling to Syria tend to gravitate towards al-Qa'ida-type groups that espouse global jihad.

The total number of British participants in the conflict is estimated to be in the "hundreds", with as many as 20 thought to have died in the fighting. Charles Farr, the Home Office's terror chief, warned recently that Brits travelling to Syria represented the "the biggest challenge" to the security services since the 2001 Twin Tower attacks"
http://www.independent.co.uk/news/world/british-fighters-in-syria-urge-others-to-join-them-9205428.html