I agree that there may not be 23000 who carry a current real threat, thank God, 'only 3000' but 20000 who potentially could be or have been in the past. Scary numbers whichever way you look at it
Found this, no idea of it's accuracy though.
http://www.westmonster.com/23000-jihadists-in-uk/
https://www.thetimes.co.uk/article/huge-scale-of-terror-threat-revealed-uk-home-to-23-000-jihadists-3zvn58mhq...