Call me nit-picky but:-
"Gawain and the Green Knight" (1973): Murray Head as Gawain and Nigel Green as the knight, directed by Stephen Weeks.
"Sword of the Valiant: The Legend of Sir Gawain and the Green Knight" (1984): Miles O'Keefe (who he?) as Gawain and Sean Connery as the knight, again directed by Stephen Weeks.
These are the only two full-on movies, but there have been other adaptations.
As to the multi-ethinicity of Roman and post-Roman Britain, undoubtedly. But archaeology now indicates that Britain was an important centre for international trade as far back as the Neolithic. Naturally, the tin deposits in the South West made Britain even more important during the Bronze Age, so we've always had people of all sizes, shapes and colours coming here.