Brian Cox is pretty famous in Britain. He tends to crop up in the better-quality TV dramas. FWIW, I thought he was better as Lecter than Anthony Hopkins and Manhunter would probably have been the better film were it not for the intrusive soundtrack.
But more to the point:
Brian Cox is Scottish and Anthony Hopkins is Welsh. Though Anthony Hopkins has only a slight Welsh accent in his natural speaking voice, Cox has a detectable Scottish accent.
Lecter/Lector’s nationality is not disclosed in either of the first two novels, so there is no reason to assume he is anything other than American, since the books are set in America. In the third book it transpires he is Latvian or Lithuanian (I forget which).
So why did two different groups of film makers cast a British actor to play him and have the actors in question, neither of whom is English, play him as an Englishman?
I think I know the answer to this one, but I’d be interested to hear other people’s views.
There’s a book of Young Frankenstein?