Are delivery robots legal on UK streets?

I have seen delivery robots pop up all over in the UK on UK streets. But are they legal?

My understanding is that such as scooters and other types are not unless they are under a certain power and speed and have pedals unless they are registered as a motorbike and have a reg plate and so on.

It just got me wondering if these delivery robots are legal to go about on UK paths or not.

If anyone is a legal person it would be interesting to know what makes them legal or not.