In 1955 De Havilland began work on the Blue Steak, a medium-range ballistic missile that would have been based in underground silos in eastern England. Powered by two Rolls-Royce RZ.2 liquid-fuelled engines (which were based on the earlier RZ.1 which themselves were based on the US Rocketdyne S-2 engine), it would have been armed with a single nuclear warhead. Several complete systems minus the warhead were live-fired at test stands at RAF Spadeadam and were shown to be very robust and reliable.
In 1960 the Blue Streak was cancelled in favor of the Douglas Skybolt air-launched ballistic missile which was to have been carried by Avro Vulcans in RAF service. Work on the Blue Streak continued as a space launcher, though, with its proven live-fire reliability an asset as a satellite launcher. However, later that year it was cancelled altogether, but all was not lost. The Blue Streak formed the basis for the Europa booster which in turn led to the Ariane launcher used by the European Space Agency.
Source: British Secret Projects, Hypersonics, Ramjets, & Missiles by Chris Gibson and Tony Buttler. Midland Publishing, 2007, p125.