Licensed/taxed as a motorcycle and sidecar. Much (relatively) cheaper than a car. A cars 'Road Tax' back in the 70s was 25 Pounds. Back then, that was a weeks salary for many. I don't remember what a Reliant's would have been, but I'm guessing less that 1/2 that.
Mr. Bean always seems to have it 'in' for the Reliant.