David is correct - very little pre-1931 Columbia metal work has been preserved. And virtually no non-classical metal work has survived - it was scrapped during World War II. However, EMI has maintained a collection of virgin Columbia and HMV shellac pressings, though I'm not aware of a similar collection of Parlophones. And, yes, the metal parts sourced for GROC LPs were destroyed after the tape transfers were made. Fortunately, RCA did preserve many of those same HMV metal parts, so it was fairly common in the 70s for RCA to 'loan' EMI metals that it had discarded.
Turns out as well that Universal, via DG, possesses about 5K pre-1914 G&T/HMV metal parts, including previously unissued plates. Historic Masters had tapped these metals for a number of their vinyl issues. Unfortunately, DG also destroyed a great deal of its own surviving metal work from the Hanover factory following World War II.
Finally, a database of the surviving EMI metal work was prepared in the 1980s while the Archives were still located at the old Hayes Factory.