The following comes from a "Summary Specification" drafted by Wardale on 17 Apr 2001:
"The power of the 4-6-0 proposal fully utilizes the available adhesion, therefore any larger locomotive designed for higher power will have to have more coupled axles. A 4-6-2 of higher power would not have adequate adhesive weight, and an 8-coupled locomotive with driving wheel diameter large enough for the envisaged speed would complicate the design process, be more expensive to manufacture, and almost certainly reduce availability. For higher power the preferred wheel arrangement would be 4-8-0, which has no precedent on Britain's railways."
Note from Alan Fozard: "Because there is no precedent for a 4-8-0 on British railways, it would be more difficult to get the rail/safety authorities to give it type/vehicle approval. The 5AT design scores because it is being evolved from the proven BR 5MT design".