Anyone can read the document as well as I can, but I think that the "special legal status" is probably a misnomer, and that they really are just stating the obvious - which is essentially the same as with Part P and BS 7671. In other words, one could tweak their statement to say:
However, BS 7671 has a special legal status. If you are prosecuted for breach of Part P of the Building Regulations, and it is proved that you did not follow the relevant provisions of BS 7671, you will need to show that you have complied with the law in some other way or a Court will find you at fault.