According to a security expert, round-the-clock surveillance of the radicalised Britons would require a team of at least 10 people to monitor each one at a hefty cost of 10,000 pounds a day.
"To track one person effectively takes at least 10, or more probably 15 people, which would cost at least 10,000 pound a day, not including other measures such as the monitoring of mobile phones and email traffic," said Colonel Richard Kemp, former head of counter terrorism at the Cabinet Office.
UK's police and intelligence services are already keeping a watch on more than 2,000 terrorist suspects as last year's 385 terror-related arrests has already been exceeded this year.
"My view is that they should not be allowed to come back. Can you imagine us letting people going out to fight with the Nazis during the Second World War back into Britain? it's ridiculous. Those people who return from Syria and Iraq are far more dangerous," Kemp said.
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Under the UK's anti-radicalisation process, psychologists are used to determine their state of mind and weekly meetings are held to determine the level of risk.
The government is also using family courts to prevent children travelling to Syria and Iraq, the report said.
"There are 2,000 individuals known to the police for terrorist activities and this excludes the 350 foreign fighters. It is a big problem," said Simon Palombi, counter-terrorism expert of the Chatham House think tank.
Speaking to the youth of the country getting attracted to the IS ideology, Cameron said, "You won't be some valued member of a movement. You are cannon fodder for them. They will use you. If you are a boy, they will brainwash you, strap bombs to your body and blow you up".
"If you are a girl, they will enslave and abuse you. That is the sick and brutal reality of ISIL. We must also de-glamorise the extremist cause, especially ISIL," he said.
"This is a group that throws people off buildings, burns them alive and its men rape underage girls, and stone innocent women to death. This isn't a pioneering movement - it is vicious, brutal, fundamentally abhorrent," he added.