I thought I might add my 2 pennies worth here. From what I've read and having talked to guys involved in the Blue Streak project in the late '50s, the original requirement that became the Belfast was for an airlifter that could carry the Blue Streak IRBM, which was cancelled in 1960. Engine tests for the rocket were done at Spadeadam, but actual flight tests were carried out (after the nuclear missile had been cancelled) from Lake Hart, Woomera, Australia as the first stage of the ELDO European satellite launcher. The Blue Streaks were shipped to Aussie, but I'd imagine that had they continued in service, they would have been flown to Aussie as much faster means of transporting them around.
I think the Shorts book mentions the Blue Streak in the chapter on the Belfast.