I guessed that but then I thought. "If they are considered safe then how can they fail". The fact that they won't pass it suggests that deep down the DVLA or whoever really don't think they are safe ( I know I don't - I drove mine on one and the handling could be best described as 'exciting' )
They are safe if used as directed but not for normal driving, as TWR has already stated their only to get you home, or to get to a garage and they are limited on speed. That's why they will fail the MOT.
Sorry dm, but my partner's car failed the MOT a couple of years ago with two nearly new front tyres being only one speed rating too low, it was a main dealers. The car had been bought 6 month previously from part of the same garage group