Rickman's accent is as commendably a good English accent for the same reason that Dolly Parton's accent is kind of East Tennessee Pigeon Forge.

What was most fun was watching the movie and attempting to understand his route.
Robin Hood arrived back in England somewhere on the south-east coast of England (white cliffs).
He then rode (presumably unlike the wind) past Nottingham and Sherwood Forest, way oop north to Hadrian's Wall, where he had a date with the men…