Only certain ones are "road legal" though
I think they basically come under the same regulations as bicycles, they should have a reflector and lights if used at night. They can't be ridden on footways, which is the difference between them and a mobility scooter.
I don't know if the government got round to classifying them, that was one of the reasons for Sinclair C5s, was to try and force the governments hand into classifying them instead of somebody going into mass production on something bigger that may be outlawed at a later date.