It's interesting (well, to me anyway!) to speculate what might have formed such a reserve had it existed. I'm pretty sure it wouldn't have been the ragbag of oddments whose scrapping was uncertain at the end of steam, which may have been one of the origins of the myth.
You'd want a mixture of classes, but priorities would be wide route availability, ease of maintenance, reliability and versatility, so I'd start with something like:
- 9Fs
- Black 5s (maybe including Standard 5s)
- Austerity tanks (from NCB or MoD?)
All of which were numerous enough to allow the best or most recently overhauled to be chosen.