15000 terrorists?
What proportion might you expect to be caught, charged and put up for trial already?
Say 3%
From 9/11 to March last year, we had over 500 here.
That would be about right for 15000.
Would you expect USA to attract less?
What our ones were doing before being caught was planning, acquiring information and resources, gaining more converts, establishing networks, keeping a low profile...
Find full stats. here.
http://www.homeoffice.gov.uk/security/terrorism-and-the-law/