I seem to remember reading in an early edition of the regulations that common sense should prevail, but that seems to have gone, and we have become students of English to work out what we can and can't do, would be interesting to read the Welsh version of BS 7671 which of course must exist or the laws in Wales could not refer to BS 7671, or it would make the law invalid. However my reading of Welsh is not very good so in real terms rather pointless asking for a copy.
Even in England it seems Welsh can be used, there was a report of a wedding which the couple wanted it done in Cornish, but it was not allowed, has to be in English or Welsh, but since in the National Eisteddfod they have Cornish speakers and Cornwell was at one point part of Wales can't see why not done in Welsh?
But still comes back to the same question, even if the regulations don't insist on RCD protection, we know morally it should be provided, yes a RCD Plug can be got for under £10, but how many would be required? It makes far more sense to have it built into the consumer unit, and only the landlord can do that.