People sneer at the Belfast(slow), but should remember that its original purpose was to ferry components of the Blue Streak rocket to Woomera
But why was it necessary to fly Blue Streaks to Woomera?
Present day Boeing Delta and Lockheed Atlas* missiles are assembled in northern Alabama and transported to KSC Florida and to Vandenberg AFB in Mexifornia via barge and ship. There is a canal and rivers which connect northern Alabama to the Gulf of Mex.
Wikipedia sez: ... The CBCs are built in Boeing's factory in Decatur, Alabama. They are then loaded onto the M/V Delta Mariner, a roll-on/roll-off cargo vessel, and shipped to either launch pad.
The Atlas and Delta missiles are probably bigger than Blue Streaks. I do not see why the Belfast had to exist in order to get Blue Streak missiles to Australia.
* ( Neither the Delta nor the Atlas is used as an ICBM. Instead, these missiles are used to launch DoD payloads into orbit. The present day Atlas is an entirely different design than the Atlas ICBM of the 1950's and '60's. The newer Atlas and the old Atlas share the same name, but not much more. The older Atlas and the Blue Streak must have been similar. Both used liquid oxygen and kerosene for propellant. )