Andrew May gurgled happily, sounding much like
they were saying:

But that doesn't explain the situation where the failure does not make
the car unroadworthy. If the tyres are illegal then it clearly is
unroadworthy as well as being an MOT failure.

But what if it fails because a headlamp bulb has blown and you would be
driving home in daylight and good visibility?

Indeed.

Perhaps an even clearer example - the windscreen on my car is currently
cracked, following a stonechip the other weekend. The crack is into the
wiper swept area by just over 40mm. It is, therefore, an MOT fail. It's
barely visible to the driver, as it's behind the mirror. It contributes
nothing to the structural strength of the car, as it's an older-style
gasket-mount windscreen, not bonded.

Is the car unroadworthy?

If I took it for an MOT this afternoon, and it failed, would it be
unroadworthy afterwards?