The trick I had to learn was that couriers like DHL do not deliver to a PO box number which is, believe it or not, allowed to be the registered address according to the Companies Register. When I deleted the PO box and just left the post code (zip code) DHL were happy with that. As David pointed out, the post code of BA in their registered address is actually the sorting office in Uxbridge, which I find to be an unusual place to have an airline HQ.