"Any attempt by you, Obama, to deny the Muslims their rights of living in safety under the Islamic caliphate will result in the bloodshed of your people" may have been the last words James Foley ever heard.

Moments later, the U.S. journalist was beheaded by ISIS militants, and the grisly video of Foley's murder was beamed around the world on YouTube. The masked man's London accent is hard to miss -- and it has ignited a debate about whether Britain, America's closest ally, is now one of the West's biggest incubators of Islamic extremism.

About 500 people linked to Britain have joined the ranks of ISIS -- the militant group that has declared an Islamic state in Syria and Iraq -- in the past several years, according to the UK's Home Office. Roughly half have now returned to Britain, prompting fears that these radicalized recruits are preparing to wage jihad against targets in the West.

Does Britain have a jihadi problem? According to CNN calculations -- based on government estimates of the number of people who have traveled from their country to Syria, and Pew Forum estimates of the number of Muslims in each country -- Britain has roughly the same proportion of ISIS recruits as France, and a much lower proportion than Australia, Belgium and a number of northern European countries.

But there is a deep concern amongst experts that Muslim extremism is a growing threat in the UK -- and as CNN's reporting reveals, there are a number of British extremists who believe ISIS' Islamic caliphate will spread across the world.

Who are ISIS' British recruits?

The video of Foley's killing wasn't just a message to America -- it was also a recruitment video for young men like Abu Bakr and Abu Anwar, foreign fighters inside Syria.

Abu Anwar is from Britain. He said he would be "more than honored" to take part in a similar act against ISIS' opponents. "I hope that Allah gives me a chance to do to James Foley to another enemy," he told CNN. "My hands are ready to commit to this blessed act."

Is there a profile for young militants like Abu Anwar? Experts paint a diverse picture of British Muslim extremists. Most are single men under the age of 30, but a significant number are older and married with children. Many are converts to Islam or are UK-born Muslims from immigrant families. Few have personal connections to known extremist figures -- and many are deepening their extremist ideology online. Some have links to gangs, but many are well-educated and middle-class.

The last time CNN spoke to Abu Bakar, he insisted he wouldn't return home, but that has now changed. Bakar appears willing to bring his jihad to British soil. "I am ready to take that step to come back if your armies, your countries don't stop attacking us," he said.

How are ISIS' British recruits being tracked?

Hundreds of British jihadis in Syria are boasting about their battlefield exploits on social media. Those accounts have been pored over by analysts at King's College in London, who are now tracking more than 450 alleged militants online.

"What's really useful about this is that you can get a sense of what weapons they're using, what they're equipped with," said Joseph Carter from the International Centre for the Study of Radicalisation. "Before, in a conflict, you would have to have intelligence you gleaned on the ground, and now you can see that stuff on Twitter."

But despite trawling through hundreds of photos and videos online, none of the jihadis the Centre has been profiling matches the man with the British accent in the video of James Foley's killing.

Even if he is found, he is still far from the reach of the British government, with no guarantee that Foley's executioner will ever face justice.

Is there support for ISIS in London?

When the world first heard the London-accented voice of the militant in the James Foley video, it spoke of Britain's long past of Islamic extremism.

This week, in a basement cafe in east London, the supporters of radical British cleric Anjem Choudary told CNN that the so-called Islamic State is not a terror haven, but a utopia to welcome.

None of the men explicitly condemned Foley's murder. One, a bearded man called Zakariyah, said that although he didn't condone the act, "if you attack someone, you should expect to be fought against" -- an apparent reference to ongoing U.S. airstrikes against ISIS targets in Iraq.

Another man, who referred to Britain as "a police state," said he would be happy to move to Syria and live under the Sharia law espoused by ISIS militants. He told CNN: "If the government would be willing to give me safe passage and not arrest me at the airport, and not raid my home ... and arrest all my family and relatives, just because they suspect I'm going there for something that they don't like -- what's wrong with going there to live under Islam?"

Choudary -- a controversial preacher whose al-Muhajiroun organization once praised the 9/11 hijackers as "the Magnificent 19," according to Reuters -- told CNN that the world had been split into two camps.

"[There's a] camp which believes that sovereignty and supremacy belongs to God. They are the Islamic State, at the head of which is Abu Bakr al-Baghdadi," Choudary said. "In the other camp you have those people who believe sovereignty and supremacy belongs to man. At the head of that camp is Barack Obama."

"I believe this Islamic State will spread, rapidly, and I believe it will be in Europe and even America within decades."

What can be done to tackle radical Muslim ideology in Britain?

In a south London mosque so full of worshipers that people are praying in the streets, the devout listen to a message of peace and piety.