This is an optimistic figure at the lower end of the estimate. The scary thing is, no one knows how many Islamic extremists are living in the U.K and how many of them are involved in terrorist activists. This is the country's blind spot, a lack of an accurate picture of the enemy so pro-active action can be taken to nip hostile activities in the bud before they take place.
(Denny Crane: "Sometimes you can only look for answers from God and failing that... and Fox News".)