Have you talked to any suppliers? I buy general purpose steel from industrial stockholders - had always assumed they wouldn't deal in small quantities, but they do, subject to a minimum order in the region £20-50. I'd imagine L6, being a rather specialist toolsteel, would be traded in relatively small quantities anyway?
+1 on Cromwell for gauge plate, but presumably as you've specified L6 you have need of some quite specific properties for whatever it is you're making.