We can say with confidence that some BR standards look like pre-nationalisation engines. But which ones?
Post a BR Standard locomotive, and the locomotive(s) Riddles might have gotten the idea from and explain how.
I think the Standard 7 is based off of the LMS's Princess Coronation class and a bit of SR Bullied Pacific. For the Standard 7 is very powerful, like a princess coronation but has the distinct look of a Merchant Navy.