The Sunday Times newspaper, citing unnamed senior government sources, reported that intelligence services MI5 and MI6 have identified the fighter suspected of killing Foley but the sources did not divulge the suspect's name.
The jihadist, who was shown on video executing Foley and then threatening to kill a second US hostage if President Barack Obama did not change course in Iraq, spoke English with a British accent.
"We're putting a lot into it and there are sophisticated technologies, voice identification and so on which people can use to check who these people are," Westmacott said.
He said as many as 500 British subjects have gone to Syria and Iraq to fight for the jihadist movement.
British foreign secretary Philip Hammond said in an article published by the Times of London that Foley's killing was an "utter betrayal of our country."
"It is horrifying to think that the perpetrator of this heinous act could have been brought up in Britain," Hammond wrote.