I would have thought that the Germans would regard the aircraft as a legitimate target.
They were sinking merchant ships by the hundred. Unless someone can find data which prooves this aircraft was targetted specifically then just another casualty of war. Compare this with the targetted shooting down of Admiral Yamamoto. Were not civil aircraft shot down on the Sweden UK run as well?
I think Lord Haw Haw might have had something to say on the matter If deliberate. I wonder if there was any Ultra traffic.