Just a reply to a couple of points raised by others.
Firstly, as far as we know, Robin Hood is a conflation of various other 'rebels'and now unknown story-tellers favourites or a purely mythical creation.
There would have been already well-known stories of adventure and derring-do re-ascribed to 'Robin Hood' as the action hero 'flavour of the medieval day'.
Therefore, adding another layer to the mythos is not a crime, it's more a continuation of the historical process - the myth of Robin Hood did not spring into being fully formed and finished, after all...
Secondly, Prince John DOES NOT SIGN anything like the Magna Carta in the movie - it actually gets destroyed!
Any complaints about the dates for the signing of the Magna Carta being out by years are therefore irrelevant, as are complaints about Robin Hood's dad writing it.
Obviously, it was just a template, later revived and improved by the English Barons : ) (Joke!)
And thirdly, as for Eleanor of Aquitaine not seeing herself as English, well, she WAS Queen consort of England from 1154-1189, 35 years, her husband being Henry, Count of Anjou and Duke of Normandy, later to become Henry 2nd OF ENGLAND.
At this point, the relationship between England and France was far more indistinct than in the modern day; great swathes of France were tied to England by marriage and conquest. If you were the monarch of a country, you would see yourself as a 'defender' of that nation, be it where you were born or not; it would be your royal duty.
Finally, the wandering accents - unless Ridley Scott did a Mel Gibson and made the whole film in Medieval Anglo Saxon English, Norman French and Medieval Latin, no accent is going to be 'authentic' - we're not even sure what the accents they had back then sounded like!
So, maybe just lighten up and enjoy this movie for the entertainment it is, i.e. not an historical documentary - although, btw, it's the only film I've ever seen that shows the correct way to take off a mail shirt (I'm a reenactor, so 'been there, done that'!)