What I'm saying is that there is no explicit law saying that fluffy dice are illegal. There is a section in the MOT that deals with obstructions to view, and that's why fluffy dice will fail an MOT, but that doesn't mean it's illegal for me to put them straight back on afterwards and carry on driving. Same for fixed buckets in a road car, steering wheels with no airbag, decats etc. Not illegal, but will fail an MOT.
You will fail an MOT if the speedo isn't reading correctly, which may be caused by rolling radius issues, but that doesn't make running tyres >3% bigger illegal.