Costner’s lack of even attempting a British accent for that movie always mystified me until recently. I read that, when the film was being made, there was another Robin Hood movie in production-—I think with Patrick Bergin and Uma Thurman. Both movies were in a race to be finished, because the first one done would be shown as a theatrical release, while the loser would be shown on TV. They filmed Costner as is to save time, planning to dub in the British accent later, but in the rush decided to skip the dubbing. Costner’s movie finished first and got the theatrical release.
I seriously doubt that Robin Hood would have had a British acent. The Saxons spoke a dialect of German, and the Norman nobility spoke a dialect of French.
And while we’re on the topic of accents, Romans rarely spoke English with a British accent.
Patrick Bergin played Richard Burton (the explorer) in “Mountains of the Moon.” He was hot.
Robin Hood himself would not have spoken with what we today call a British accent. He would probably have spoken Anglo-Saxon, a language as different from modern English as modern English is from modern German, Dutch or Swedish.