"...hot British sub machine gun ammo..." No such thing. The Commonwealth used the same ammo in SMG's and pistols. Mostly Inglis or FN made BHP's. In war time there was no logistics time to fiddle around with 2 kinds if the same thing.
9mm NATO is considered +P by SAAMI though. Your Walther will shoot that with no fuss. 9mm NATO doesn't bother post war P38 with the Al frame. That being what was issued to the 1950's German Army.
However, W.W. II vintage ammo of any flavour is collector grade stuff when it's in the sealed original boxes. You may want to look into that aspect before shooting it.
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