Important to note is that the inside of a tyre contains ply and cords which can become exposed if the tyre has a deep cut in the tread or sidewall.
According to UK road law, if a tyre cut is 25mm or 10% of section width – whichever is greater – then these are grounds for instant MOT (Ministry of Transport) test failure. A cut of this size is considered deep enough to reach the ply and cords.
If you discover a cut on the tyre surface before your MOT test, either on the sidewall or the tread, then it's advisable to replace it with a new tyre. An attempt to have it repaired is not recommended, and for the sake of tyre safety it mustn't be ignored.