It would not surprise me if the scanned in the mugshot books and not only have photos going back for years but have multiple pics of the same people taken over the years. Likewise there's all those covert photos taken at raves, riots, protest, outdoor concerts etc.
Having 18m photos is not the same has having had 18m people arrested and processed through the system. Not only that but the "facial search" doesn't sound as though it's especially good anyway. It does sound as though they are retaining far too many images of unconvicted people though.
There was a program, on BBC R4 (PM?) a while back talking about (IIRC) "super recognisers",a smallish but significant group of people in a certain age range who seem to be incredibly good at remembering and recognising faces, even from poor photos, who are far, far better than any current computer system and they are being actively searched out and recruited.