I know you guys are in UK, but maybe you know about this issue.
According to the m2 guide there should be a button that says "Create Shipping Label " when we enter the “ship” dashboard for an order.
My m2 is setup with USPS, it calculates shipping cost and everything but when I click the print button it just download the packing slip.
I have asked about this for months now on magento forums, reddit etc no one knows how this works, people just say use a third party service.
I asked usps for that third level API approval, but am not even sure what to do after that