Have more British Muslims joined Islamist groups than serve in British armed forces?

Question

From the New York Times: Her Majesty’s Jihadists

Her Majesty’s Jihadists

More British Muslims have joined Islamist militant groups than serve in the country’s armed forces. How to understand the pull of jihad.

and from the body of the article:

Ifthekar’s story would become an iconic one of the foreign jihad in Syria. It was recounted to me by Shiraz Maher, a senior research fellow at the International Center for the Study of Radicalization (I.C.S.R.), an innovative institute at King’s College London. Here, a handful of researchers have been charting, following and, in some cases, interacting directly with foreign fighters in Syria and Iraq — through text-messaging and smartphone apps — in hopes of understanding their motivations and their worldview. The center now monitors some 700 of the 20,000 foreign fighters from 90 countries around the world. (Foreigners make up half of ISIS’s total fighting force.) An estimated 4,000 are from Western nations, some 600 to 700 from Britain alone. More British Muslim men have joined ISIS and the Nusra Front than are serving in the British armed forces.

Have more British Muslim men joined the Islamist groups ISIS and Nusra Front than are serving the British armed forces?

Answer 1

The figures vary. There may or may not be more British Muslims serving Britain than Islamist groups, but figures indicate that Muslims are under-represented in the armed forces.

From The Guardian, British army aims to recruit more Muslims after worries over low numbers (talking about the army, not the armed forces as a whole)

There are 480 Muslims in the army out of an overall strength of 88,500 – 0.54%, an increase from 300 in 2008. Muslims comprise about 4.4% of the UK population according to the 2011 census.

An opinion piece Muslims in the armed forces: A proud tradition

Despite this, the figures in absolute terms are still small, with around 650 Muslims in the 200,000-strong military making up less than 1 per cent of its ratio strength.

From the BBC: UK's Muslim soldiers ‘fighting extremists not Muslims'

With more than 600 Muslims in the British Armed Forces, do those that are deployed on the front line in Afghanistan have to reconcile their beliefs in order to fight hardline Islamic Taliban militants?

Caveats:

• I haven't critically examined the number of British Muslims serving Islamist groups.
• Could estimates about the number of British Muslims in the armed forces be incorrect? Maybe there's people who are Muslim, but haven't mentioned their religious affiliation to those collecting statistics.
• Some of those serving the British Armed Forces may be women. It's possible the Islamists have more men, but the British Armed Forces have more people overall.