Blue Envoy was to be a long range surface to air missile, intended to defend the UK from massed Soviet bombers. It was to be powered by a pair of Bristol BRJ.811 ramjets, with 4 Borzoi solid rockets to boost it to the speed where the ramjets could be used, giving it a range of around 150 miles. The warhead was to be a large highe explosive device, but it could have been fitted with a nuclear warhead later.
Blue Envoy was cancelled in 1957, but much of the work done on the guidance radar and ramjets found it's way into the Bristol Bloodhound II, which remained in RAF service into the 1990's.
Many thanks to Chris Gibson for his research into this project.