Daily Telegraph last week.

"British Islamists have travelled abroad to fight before in recent years, including to Chechnya, Bosnia, Afghanistan and Libya. But Syria has attracted more foreign jihadists than any previous conflict, with Britain supplying the greatest number of recruits.
To put Mr Cameron's warning into context, we should consider that over the past 10 years 34 terrorist plots connected to Pakistan have been foiled in Britain. Several would have caused mass casualties had they succeeded. The security services think the potential threat from Isis is greater. And, in any case, as the July 7 bombings in London showed in 2005, it only needs four extremists prepared to commit suicide to cause carnage on the transport system of Europe's biggest city.
In Syria and now Iraq, al-Qaeda recruiters are on the look-out for the next generation of terrorists. Finding out who they are is the main preoccupation of British intelligence."
http://www.telegraph.co.uk/news/uknews/terrorism-in-the-uk/10914803/Are-the-British-jihadists-going-to-turn-their-guns-on-us.html