(CNN) -

It was a gruesome act performed atop the stage of the global theater -- the grotesque image of a masked man, dressed all in black, beheading an American journalist in a production intended to strike terror into the hearts of millions around the world.

This act of sadism was horrific enough on its own. But what some will also find deeply disturbing was that the jihadist executioner communicated his threat with a distinct London accent. The realization that there are people who grew up in Britain who are prepared to engage in such barbaric acts of depravity makes James Foley's murder feel more intimate than if it was perpetrated by a foreign-sounding killer from a different society.

Propaganda films from ISIS -- the Sunni militant group that has seized large tracts of land in Syria and Iraq in recent months -- regularly feature British recruits to demonstrate the group's capacity to influence young Muslims living in Europe. Earlier this week another threatening video from ISIS featured a group of jihadists speaking with British accents as they interrogated a Japanese hostage. Some of them weren't even hiding their faces.

The beheading of Foley was staged as a "Message to America," but it constituted a direct warning to Britain. It served as a reminder that the killing of a young English soldier Lee Rigby on the streets of south London by two home-grown jihadists last year was not an isolated event.

There has been a dramatic shift since September 11, 2001 in the way that the risk of terrorism is perceived in Britain and Western societies. Western governments have been forced to confront an unexpected and deeply disturbing reality -- that it is sometimes the people already living in these societies who constitute the greatest security threat.

The emergence of the "home grown terrorist" raises the fundamental question -- why do these radicalized jihadists reject the values and ways of life of the societies they inhabit?

Thankfully only a small fraction of a minority of young radical Muslims turn into hardened executioners of innocent victims. But a far greater number reject, even loathe, the cultural values of British society.

Many radical Muslims aren't fervent ISIS supports -- but some do regard the war to establish a global caliphate as a cause worth supporting. Their response is integral to an uncomfortable reality that British society ignores at its peril.

Losing the battle of ideas

Since the terrorist bombings in London in July 2005, the challenge of winning hearts and minds has been evident to policy makers. At the time, British Prime Minister Tony Blair declared:

"It's important, however, that those engaged in terrorism realize that our determination to defend our values and our way of life is greater than their determination to cause death and destruction to innocent people in a desire to impose extremism on the world."

Unfortunately very little progress has been made in upholding and explaining the values and way of life that are at stake -- and the shallowness of this statement was exposed a few years year later when the government's plans to launch a British Day had to be quietly abandoned.

The idea for organizing a British Day was a direct response to the London bombings. At the time, Chancellor Gordon Brown stated:

"We have to face uncomfortable facts that while the British response to July 7th was remarkable, they were British citizens, British born apparently integrated into our communities, who were prepared to maim and kill fellow British citizens irrespective of their religion.

"We have to be clearer now about how diverse cultures which inevitably contain differences can find the essential common purpose also without which no society can flourish."

Sadly the government failed to give meaning to the idea of this "common purpose" and gave up on the idea. The very attempt to celebrate "Britishness" only revealed an absence of clarity of what it was that ought to be valued and celebrated.

The answer to the question of what it means to be British continues to elude policy makers. Prime Minister David Cameron has called for teaching Britishness in school, in response to recent allegations about radical Islamist influence in the classroom. But if political leaders find it difficult to explain what Britishness represents, then how can teachers be expected to instruct their pupils?

Unless British values actually mean something in public life they cannot be taught. This is a challenge that has been evaded during the past decade. After the tragic murder of James Foley, this challenge must no longer be avoided.