Ok, Im not really sure about infrastructure at ocuk and what has already been suggested but how difficult would it be given your current ERP system to generate individual and unique codes and email them to every OCUK forum member who would opt in and be interested in such deal.
What I mean by that is lets say for 3070 we would have 100k+ different GUIDs each one send to different forum member, when someone decides to buy 3070 during checkout process we would remove that GUID from lists of available valid voucher codes, this would ensure even if someone leaks it wouldn't really work. IF we want to be super strict we could tie each code against user details.
If we want to avoid mass spam ocuk could create landing page where you have to log in using your forum details, and once logged it would generate your unique discount codes