It is questionable because it implies that the street belongs to St. James. Clearly it doesn't, so if it is merely named after him, it should be just St. James Street.I agree, other wise St James's Street would be bad grammar, which it isn't.
!0/10 here, too. Like Super Steve Earle, I also went to a Grammar School (sorry, I know that for a lot of posters those are two very dirty words). I wonder how many more of the high scorers had such an education