I think from memory that they were taxed (registration) the same as a motorcycle and sidecar......Welding the diff up will do that to a car.
I remember reading a period test on the new 1961 Regal in (I think it was) The Motor Cycle.
They tried numerous times to roll it and only succeeded after applying full lock at 30mph.
Three wheel cars in England were usually tested by bike magazines as they could be ridden on a bike licence and cost much the same as a sidecar outfit.