Craig,
Steel in England comes in two different levels of proof application:

[Linked Image from i.ibb.co]

Guns that have the current version of Nitro Proof (revised early 2000s, I’m thinking, one of the English guys will straighten me out if I’m off) are OK with the “Standard Loading”, guns with the ace of clubs looking thing, or marked “Steel Shot”, can use the “High Performance” steel loading. That load is closer to what is on the shelf here in the US.

I have an issue calling any steel loading high performance. The steel shot stamp would seem to be easy to fake, to me.

YMMV.

Best,
Ted