I can give you my opinion until someone more knowledgeable contributes. Do not take the Autosomal DNA test for the ethnicity piece alone. It not only does not break out Eng, Ire, Scotland, but also includes Normandy & Brittany with the British Isles. This makes sense but won't help you.

The DNA test will match you with biological cousins. Even if your father's immediate siblings and cousins from that side do not test, you may get some good clues as to his father's identity. You may have to do some digging, although it is possible that a surname search of the results might give you the information that you need.

Best of luck,
Kathy