The point is that the law already bans privately owned e-scooters for usage in public places. It also already requires e-bike riders with >250 watts (continuous output) motors or those capable of motor assistance/sole propulsion above 15.5 mph to be 16+, have at least an AM (CBT) driving license, DVSA vehicle model approval, third-party insurance, MOT and a number plate when used on public roads. The problem is that the current laws regarding these issues are rarely enforced, it is easy for people to buy these road-use illegal/regulated vehicles relatively cheaply and to conceal them at home due to their small size.
The Police are under-manned and obviously have to prioritise which road traffic offences they try to prevent/investigate. The damage that an e-scooter or e-bike which is illegally ridden on the road can do is clearly a lot less than a car/van/lorry/truck can do, so understandably the Police will concentrate on the latter group of vehicles.
If you feel so strongly about it then when you see someone riding an illegal e-scooter or a clearly unrestricted e-bike (not a pedelec) in a public place then make a video of them doing it and follow them home. Give the Police their address and the video and report them for a road traffic offence. If you see someone riding a pedelec, bicycle or rental e-scooter dangerously then you could do the same thing. (For the rental e-scooter you may not need their address as the company who rented them it should have a copy of their driving license on file and it should be able to see where the e-scooter has been ridden via its GPS tracker.)
If you see someone riding dangerously on one of these vehicles, or a bicycle, who causes an accident then you can do a "citizen's arrest" on them and detain them until the Police arrive. (I would not recommend doing a citizen's arrest on someone who is just riding an illegal e-scooter in a public place though, although if he was driving it dangerously and you have video evidence of that then it might be OK. The Police could charge you with "Wasting Police Time", or worse, if you call them out after doing a citizen's arrest on someone for something the Police would not have arrested them for. Also, a citizen's arrest would be far safer legally if you wore a body-cam and filmed it because if you need to defend yourself you would have evidence that you did not assault the arrested person.)