The Vantrue N2 Pro Dual lens cam also has a 'cabin cam' as well as the standard dashcam, and this image from Amazon suggests that to all intents and purposes it's a CCTV system.
Interesting that Nextbase (which I'm assuming is the market leader for dashcams) does three different back-facing add-on modules for its standard dashcams:
1 The 'rear window cam', which is mounted in the rear window. Obviously that wouldn't make the system CCTV.
2 A 'cabin view camera'. That's similar to the Vantrue above, thus that would make it a CCTV system from the regulatory perspective, I would imagine.
3 The other module is maybe the more tricky one, though. It's mounted on the front windscreen as well, but is called a 'rear view camera', and it's pointing at the road behind the car rather than on the cabin and passengers.
But it would certainly catch any centre back seat passenger (who in turn would block the view of the road behind), although that would depend on the make of car and where precisely the camera was pointing.
You can get an idea of what's going on from the graphics here:
https://www.nextbase.com/en-gb/modules/Another thing of course is that I think most dashcams can record audio, so even if it's not pointing inwards there could still be a regulatory dimension if the audio is enabled.
Incidentally, looks like Nextbase doing 25% off this weekend, so worth a look if anyone thinking of buying. A mid-range cam with a rear-facing module (all three rear-facing modules are the same price) and hard-wire kit (essential for a taxi, I suspect) would be around £150, so cheaper than from Halfords or Amazon.
(Of course, there are no doubt cheaper and better dashcams around, but no doubt there's a reason Nextbase seem to shift so much kit.)
https://www.nextbase.com/en-gb/