You can buy them in the UK, BUT you CAN'T ride them on a UK public road, cycle lane or pavement. It is an offence to do so, and people are being issued with fines and even being given points on their driving licences!! Be careful.
The only place an e-scooter can be used is on private land, with the permission of the landowner. No Council has provided permission for privately owned e-scooters.
They are currently classed as Personal Light Electric Vehicles (PLEVs), and are treated as motor vehicles - subject to all the same legal requirements - MOT, tax, licensing and specific construction.
As such, because they don't have visible rear red lights, number plates and a signalling ability they can't be used legally on the roads.
E-scooters and the law (pdf) from the police.
Advice on e-scooter usage from the police.