Here is evidence that the numbers have not gone down.
It is provided for us by MI5.

Our experience over the last ten years has shown that networks of terrorist supporters can be extraordinarily determined, resilient and patient. Some groups that have been disrupted or have had members convicted of terrorist or other offences have nonetheless been able to recover. They have resumed terrorist-related activities within a relatively short period of time and sometimes under new leadership.

Over 310 individuals have been convicted of terrorist offences in the UK between 11 September 2001 and 30 September 2012. A number of those who have been imprisoned and are due for release are still committed to extremism. They are likely to return to terrorist activities and will need to be monitored in coming years.

Experience has shown that it is very rarely the case that anyone who has been closely involved with terrorist-related activity can be safely taken off our list of potentially dangerous individuals.

Terrorists continue to aspire to attack high-profile targets in the UK and abroad. Major public events present new opportunities for terrorists. The UK's police and intelligence services continue to step up efforts to counter the ongoing threat from international terrorism.
https://www.mi5.gov.uk/home/the-threats/terrorism/international-terrorism/international-terrorism-and-the-uk/the-trajectory-of-t