I believe it's the same law [1] that also meant the Hoverboards from a few years ago were also illegal to ride on public pavements and roads[2].

[1] http://www.legislation.gov.uk/ukpga/Will4/5-6/50/section/72

[2] https://www.theguardian.com/uk-news/2015/oct/12/uk-hoverboar...