It's not really anything to do with any 'regulation' but, rather, that the product Standard for devices in CUs do not force Standard-compliant products to be interchangeable, regardless of 'make' (as is the case, for example, with "plugs to BS1363").This reg, no doubt was made to happen, by the manufacturers who have undue influence over JPEL/64, obviously worried about 3rd party products costing them profit.
As previously said, I strongly suspect that 'commercial pressures' were behind it - and I fear that the trade organisations which one might have hoped would resist this (on behalf of electricians and consumers) were also under the same 'commercial pressures'