In answer to my question, here's a quote from Dave Thaxter's book '
The History of British Caledonian Airways' (available for purchase through this link):
July 1966 saw plans for a new livery for BUA’s fleet finalised. The sandstone and blue livery was given to VC10 G-ASIX first, which flew the new colours on 11th August to South America. Next, G-ASIW was completed at Gatwick by 1st September and one of the Africargo Britannias was rolled out on the same day by Aviation Traders.
November saw the first 1-11 fly in the new colours. The new livery was applied by “Buster” Brown and his team at Gatwick, which included three painters and seven assistants. They could complete a VC10 repaint in seven days using just 30 gallons of paint.
The photo of G-ASIX on his website is undated, but given the information above there's a fair chance that it was taken around July or early August of 1966.