Sean Bean's natural Yorkshire accent gives his character a bit of authenticity and a kind of northern ruggedness that sort of Sean Bean accent like that. Now Boromir could have spoken with RP like the other high level characters like Aragorn but he didn't want to do it. In the film Boromir's family are the stewards of Gondor they're basically there to kind of look after the kingdom ofGondor while the proper royal family is not around.