A small order from Plastruct would probably be a good starting point.
They have all sorts of scale pipefittings, valves, different textured sheets as well as H, I, C and other common industrial steel profiles in various sizes.
For doing bases, a bakers dozen of different profiles, plus a handful of treadplate textured sheets and a dozen or so valves should be plenty to do a whole army and have a bit left over at the end.
You can save a bit by getting more generic items from less expensive suppliers (Evergreen is almost half the price of Plastruct for many items...) and then just pick up the special bits from Plastruct...but there are shipping and other issues in play there.
Most model train shops should be well suited to supply these items (either in the above brands or something more specific to the UK).
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