Well, that's not necessarily correct. It's all about the concept. Is a car with a wrongly spaced number plate "unfit for the road"? The relevant legal concept is "roadworthy".
There's a major difference between MOT fail and unroadworthy. The two concepts are covered by different legislation although they appear to and can overlap.
MOT legislation says that IF a car needs an MOT it must have a pass certificate that's less then 12 months old. So if you car fails an MOT when you've got 6 months left to run on the old MOT you still have a "valid MOT". Similarly, if a car doesn't need an MOT at all then an MOT fail is, in itself, irrelevant. You are still compliant with the MOT legislation.
Separately from the above there's a completely different law which requires any car on the road to be "roadworthy". Having a current MOT doesn't mean your car is roadworthy and similarly not having a valid MOT (where required) doesn't mean you car is "unroadworthy".
It's slightly bizarre the way these two laws interact but that's the way it is. In a nutshell,
* One is an administrative piece of paper. If you haven't got the piece of paper you're guilty, regardless of the perfect condition of your car.
* The other is a real-world assessment of roadworthiness and having an MOT doesn't mean you car will be "roadworthy" for the next 12 months.
OK, OK, so what is "roadworthy"? See below,
"If you want a legal definition of what renders a vehicle unroadworthy then check out the Road Traffic Act 1988.
Road Traffic Act 1988