Boromir's family are not part of the same lineage as the proper king of Gondor and so they don't have the same level of stature. So Boromir's Yorkshire accent is quite appropriate because he's not you know quite as upper class as as characters like Gandalf and Aragorn or these high elves. For me Gimli's accent in the film sort of reflects the harshness of the of the dwarves but also a lot of the warmth of them as well. In for example the Hobbit movies the dwarves all have local accents including Thorin who's their leader with received pronunciation.