"B.I.L Steels Ltd, Eyre Street, Birmingham, B18 7AA are suppliers to the Heritage panels pressing plant and will sell you sheets of the same material if you collect it.
https://www.bilsteels.co.uk/sheet/
The steel is 1mm, 19gauge and designated DC04.
This is a very malleable steel specially produced for "deep drawing" pressings.
Don't ask for small pieces as they don't cut to size but I believe he might supply half sheets and may now have slightly thicker sheet."
From this original post on TMF.
Body Steel Supply:-
https://www.theminiforum.co.uk/forums/t ... el-supply/