Heaven help you if you have a motorbike as heavy as a Morgan three wheeler!
Especially if it only has a centre stand!
Seriously though, it strikes me that vehicles like the M3W sneak around lots of legislation because the bureaucrats simply had no idea that anyone was likely to go back to building something akin to a cyclecar. Sticking the M3W's head above the parapet to argue about the nuances of licensing might just bring it attention that we don't want. Once the bureaucrats realise that it is out there they might want to put a stop to it.
On a self interest note I could add that as I have a full car and bike license and an M3W, I'm all right Jack!
As an aside, a Police Sergeant who was looking at my M3W did ask how they got it past the pedestrian protection rules, he seemed to appreciate my reply that it was just designed to cauterise any wounds at the point of impact.