The message from John Rumm contains these words:
I don't think they have changed (other than the units) for a good bit longer than that.
I can't find a single copy of The Highway Code atm (I have 3 stretching back over the later half of my driving experience) but ISTR that the formula is 0.5 seconds thinking distance plus retardation at two thirds g. I wouldn't doubt halving the thinking distance but retardation well in excess of g from an ordinary road car seems a little suspect to me. IIRC brake efficiency for cars only needs to be 50% so was it that that was doubled?