When I made my Drum Car I looked a little into the regulations and designed it to conform (vaguely!) to the rules governing Electric Bicycles and Tri-cycles. Despite having Horizontally Opposed Twin electric motors, its power is modest, and down-geared, so that it cannot get up to the 18 kph limit, (except when going down steps!).
Although it differs somewhat form electric bikes, if I ever do get stopped by the Police I can demonstrate that its performance is well within legal limits. This means that it is effectively legal on roads, pavements, parks, civic centres, supermarkets and shops, all of which it HAS driven in!