What the authorities cannot predict are the derogations that will be need to be granted in each instance where the 5AT does not meet the standards, especially given that the standards were not written with steam locomotives in mind. Notwithstanding, the swift approval that was granted for the A1 Project's Tornado to run on the UK main line suggests that it should be equally possible to gain approval for a design that is so similar in form to the BR Standard 5MTs that currently operate under their "grandfather rights"
The reality is (for the 5AT as it was for the A1) that no guarantee of approval can be offered before the design work commences. The approval process requires the project "owner" to appoint a registered Vehicle Acceptance Body (VAB) to monitor the detail design of the locomotive and to verify that each step of the design complies with (and to record each non-compliance with) its relevant regulation.
VAB endorsement is the principal factor governing rail authority rejection or approval. A VAB cannot offer any sort of endorsement until the design and construction of the prototype locomotive is completed.