If Father Douglas was found guilty, not of being a Catholic priest in England, but of "persuading to popery" under the 1581 “Act to retain the Queen’s Majesty’s Subjects in their due Obedience”, authorities must not have been able or thought it not necessary to prove that their prisoner was a Catholic priest. If they had proved him guilty of the latter, he was would have been sentenced as a traitor under the 1585 "An act against Jesuits, seminary priests, and such other like disobedient persons." Finding him guilty of bringing others to Catholicism was also to find him guilty of treason and thus he was hung, drawn, and quartered in York.
Note that he was born in Scotland but served Catholics in England!
Blessed George Douglas, pray for us!