Flightwatch #17 has it, apparently the story is that the huge increase in TBO on the Dart engine on the V802 in the late fifties left BEA with a big surplus of spare engines which they had fitted to their new Argosies. These proved underpowered hence were fitted with engines pulled from the 6 V806 Viscounts which became the V806X (fitted with Darts from the V802 spares pool). This all presumably a few years before BKS/BAS got the V806X.
As an aside our honeymoon was a Cambrian BAC-111 red-top 1 week IT to Marbella/Malaga (from Heathrow ,too) in Feb 1973 seem to remember it was £35 each :-)....also that year I think BEA launched some ultra-cheap fares to Paris, Amsterdam (and Brussels)...seem to remember £13 one-way with some travel/booking time restrictions...anyone remember it?