I've had exactly same situation with customs when I ordered last year from Aero to Poland. I knew about "Goods manufactured in the UK are exempt from customs duty in the EU countries." case but DHL added duty to my order. I've contacted DHL and they said it was accounted because there was no 'certificate of origin' attached to the package. I've called Aero and they emailed me the certificate, I then forwarded it to DHL and they cancelled the duty. Maybe they lost it somewhere but if I didn't know about that case I would overpay a lot.