Raul's played British characters many times before: The Homecoming, as you mentioned, Chitty, Taboo. He actually did Arcadia a long time ago in Chicago -- he played Septimus. Anyway, all of the Brits I've seen him play have had slightly different accents, because of background, time, or place, and some have been better than others (mostly because of the crazy speech thing he was doing with Philip Salon), but for the most part he's fine. I think the particular accent for Arcadia will be different from those other ones, though, since, of course, there isn't really just one standard/generic British accent. I don't think it's anything to worry about.
I will be out of town when the marquees and whatnot go up, and can't see this until after it opens. I'm going to be that person and say that someone has to promise me they'll take and post photos of the theater. Putting that on record now.
A work of art is an invitation to love.