Commentary in response to the Europol EU Terrorism Situation and Trend Report linked at the top of the thread:

GEES, 16 Jul 07: Europe and the Nature of the Terrorist Threat in 2007
...It is worth noting in considering these facts and figures that Europol is not privy to the full range of information available from member states on terrorist attacks and incidents, primarily because some states such as the UK are reluctant to share details of ongoing operations, including arrests of suspects. The recently departed Director-General of the UK’s Security Service has suggested there are approximately 200 active Islamist jihadist networks active in Britain, comprising 30 known plots, and involving 1600 identified individuals. These statistics should be considered in con-junction with those made available to Europol.....