Basically, in UK law, the scooters count as motor vehicles, and therefore must have a licence, which they can't for other reasons. So, they're illegal to ride on the pavement because they're motorised, and they're illegal to ride on the road because they're uninsured and unlicensed. There are a few trial areas where companies are providing insured scooters for hire under a recognised scheme, which is what makes them legal.
This is the same reason why electric bicycles in the UK must only apply power to assist you as you pedal - they must detect when you pedal, and act as power-assisted pedalling. It's illegal to have an electric bike that gives power when you're not pedalling, because that would count as an unlicensed uninsured motor vehicle.
It's a quirk of the way that classes of vehicle are defined in UK law, and I'd rather they just sorted it out.
It sounds like these issues are artificial legal constructs caused by the law lagging behind where technology js, and it would be trivial to fix them.
For instance, I'm sure insurance companies would be eager to offer escooter insurance.
What's the rationale behind this nonsense?