I haven't seen The Eagle, but I liked Centurion
Channing Tatum is in The Eagle, so I'm not really rushing to see it or expect serious acting there.
I like the British villainy sounds, too, coughseanbeancough. And Fassbender has previously also done Eastern European type accents, so maybe thought a vintage Polish accent would be too unappealing to the target American audience today?
Fassbender and McAvoy both did interviews talking about hoping to have sequels where they can build up to what we expect in X and Magneto. MF said he wanted to graduate from the constipation hands to the effortless style of Sir Ian. Maybe if there is a sequel we can more stylized operatic deaths or something.