The news that British Prime Minister David Cameron has cut short his vacation after the video of an alleged British subject beheading Irish American journalist James Foley became public is certainly dramatic.

Britain suddenly finds itself with an 'enemy within' – the almost 500 citizens who have left the UK to fight for a caliphate or an Islamic world order.

The beheader of James Foley was clearly one of those, speaking in an obvious London accent. Experts say the British fighters are among the most extreme.

Jihadi expert Shiraz Maher, a senior research fellow at London University, told the "Today" radio program, "Unfortunately the British participation in the conflicts now raging in both Syria and Iraq has been one of full participation, one that has seen them at the front lines, taking part in the conflict in every way.

"So we have seen British fighters out there operating as suicide bombers; we have seen them operating as executioners.

"Unfortunately, they are amongst some of the most vicious and vociferous fighters who are out there. That is unfortunately just a part of their radicalization."

Guardian columnist Michael White says there is a perception that Muslim leaders like Catholic leaders at the time of the IRA campaign were not doing enough

“As with the IRA during the Troubles there is usually a thread of ambiguity among some co-religionists, priests and mullahs, which the media prefer to focus on.”

The father of one of the known British fanatics, 20-year-old Nasser Muthana, said his son was lost to him.

Ahmed Muthana said his son had wanted to be a doctor, but chose "to go with these wrong people."

"I think, 'Am I going to see him alive again?' Maybe we won't even see the coffin – we'll just see on the news they're dead," he said.

Bizarre that, the contrast between a young man one day thinking of becoming a doctor, the next a Jihadi fanatic and then choosing the latter.

Another British terrorist – Abu Osama – told the BBC he utterly rejected his home country.

He told the Guardian that Britain was "pure evil," and said he would only return "to raise the black flag of Islam over Downing Street, over Buckingham Palace, over Tower Bridge and over Big Ben."

Even young women have taken to Jihad. Manchester twin sisters Salma and Zahra Halane are said to have traveled to Syria in June. They were described as "deeply religious."

Many Muslim leaders are doing their best to stem the tide. The Guardian reports that more than 100 imams have called on British Muslims not to travel to Syria and Iraq and have written an open letter urging local communities "to continue the generous and tireless effort to support all of those affected by the crisis in Syria and unfolding events in Iraq." but to do so "from the UK in a safe and responsible way."

Judging by the fanatic hearts of those who have already left it may be too late.