IIRC the deal with three wheel vehicles was they could be registered as motorcycles and the registration fees and taxes were much lower.
At least the Germans remembered their childhood tricycle.
The Brits also made a rear engine car that had only one door. The entire front of the vehicle swung out (up I believe) which was problematic if someone parked too close to you.
Hard to imagine the country that revolutionized aircraft with the Spitfire, took the war to the Germans with the Mosquito (the Wooden Wonder), developed the RR Sterling engine and kept the seas free of NAZI warcraft would stumble to such lows as the Robin and Lucas Electrics. :-(
Yes, I think that was it. The UK had hundreds of motorcycle sidecars, and give the weather many were enclosed...so that might have been a generous grandfather clause allowing for the Robin.
Though the strange British Labour party mania for taxing everything allowed for kit cars such as the Lotus 7 to escape the most onerous taxes. The Early Morgans roadsters were also three wheelers with motorcycle engines to slip under the tax definitions of an auto car as a motorcycle.