For me, it is Astin's performance accent-wise that is the weakest. I particularly hark to the line after Weathertop, "What are ya doin'?! Those wraiths are still out there!" Not a Yorkshire-burr by any stretch. Much more...Californian...
As for the other non-Brits, I think Mortenson does rather well, Wood is steady if overall unconvincing, and Tyler just sounds like she has her mouth full. I won't even mention Agent Elrond, because it's just too painful.
