Copy & paste from the MOT Manual, which may help clarify the question …
The following criteria should be used when assessing a cut in a tyre:
any ply or cord that can be seen without touching the tyre – fail
[b]if by folding back rubber or opening a cut with a blunt instrument, so as not to cause further damage, exposed ply or cord can be seen irrespective of the size of the cut – fail[/b]
if a cut which is more than 25mm or 10% of the section width whichever is the greater, is opened with a blunt instrument and cords can be felt but not seen – fail
Before failing a cut, you must make sure it’s the cords that you can feel not a foreign object. If you’re not sure, then you should pass and advise.
Dificult to give a definitive answer based on a photo of the cut in question, but if ply or cord could/can be seen when the cut is opened as described above, then the size of the cut is irrelevant & the tyre would fail.
