An additional point - if you receive metal back rather than money, then that still gets declared. Your books should show value for raw materials held as well as stock, so the addition to that will show up.

You wouldn't believe how much computing power HMRC now has at its disposal - and if they decide you're defrauding the tax payer (with whatever pattern they decide constitutes evidence) then they can shut you down very quickly and with remarkably little judicial oversight.