Due to our best price policy we do not publish any discount codes, and never have.
We instead have a best price policy that's offered to everyone. 
Any codes or vouchers you may have seen published on 3rd party sites are simply made-up and are certainly not valid
(That's why we don't have a voucher box in the checkout)
Why do they do that? Many voucher sites simple make-up false codes to encourage people to click on a link where they can then place tracking cookies on their computer.