I did this once over ten years ago and from Spain not England so things may have changed. Bought and paid for two guns in Barcelona contacted my FFL to fill out form 6. You do need an FFL to help you. He knew nothing about a form 6 so I filled it out and he signed it. 4 weeks later we receive the approved form 6 back from the ATF. Spanish shop arranged shipment but only to Kennedy. I hire freight forwarder to take to Kansas where I was living at the time. If you have a customs office locally you can clear yourself easily. I made sure that the packaged is bonded all the way to Kansas so customs won't touch/stop/open it in New York. I send/fax a copy of the approved form 6 to Spain. A few days later I get a call from the local customs agent saying he has a package for me. I call my FFL and we drive to the customs office. The three of us walk over to the freight forwarder. Agent checks the guns' serial number against ATF form 6, we walk back to customs where he signs the import paperwork. No duties or fees were charged. They were beretta SO's so were modern guns. As anything involving humans, your experience will likely be different. I think this guy didn't charge the import duty because he didn't know any better. We took the signed import paperwork to the forwarder, picked up guns and drove to FFL store where he transferred them to me. Easy peasy
