M Machine are in Darlington if that’s not to far, I used them oncea when working in that area to buy some big chunks of En1A.
Now living in Aberdeenshire local suppliers are thin on the ground so I buy from GLR and Live Steam Models both of whom advertise in ME and give good service.
If your OK wthj mild steel black bar your local blacksmith will supply or, if your prepared to buy full lengths, usually 3m, your local metal supplier may sell to you. For sheet metal go to a local sheet metal shop, they’ve plenty offcuts they will let you have.
Nigel