Boss
I think it is easy to forget how much less understanding everyone had about high level and high mach number in the 40s when the replacement for the Lincoln was being specified. As has been said it was about not falling on our face. The Sperrin was a very conservative design with respect to the three major development issues of aerodynamics, structural design and power plant installation - so really unlikely to fail, but because of this a long way short of the performance wish lists of the day.
The Valiant, by comparison, was quite advanced compared to the Sperrin and a really staggering leap from the Lincoln.
The spec that produced the Vulcan and Victor (B.35/46 issued in 1946!) really was stretching the available knowledge of the day as far as it could go, indeed so far that failure was a very real possibility. Because of this both very different aero concepts (delta and aeroisoclinic wings) used small prototypes to give their design teams a degree of confidence (technology demonstrators had not been invented then).
Another way of looking at it was that nobody with any appreciation of the problems in the late 40s would have bet that all three V design teams would have succeeded. It was really quite remarkable that they did.