My Zed (built late 1979) would technically still qualify for the old black & white plates, but I think it would look ridiculous.
Our family car bought new in 1971 (Hillman Avenger) had the raised black plastic lettering as your Mk1. I remember my dad swapping the rear yellow with black plastic lettering one for a flat 'new style' reflective one, I guess there must have been a change in the MOT requirement. I think anyone buying a new car back then would have seen the old black and white ones as dated.
As a side note; anyone remember these stamped ally horrors from the 80s? I binned this one off the front of the zed. This style always reminds me of Ford Sierras for some reason? This would be the period correct look for my motor, but not a nice one.