I always pictured Rupert Penry Jones as RCB.Only thing I've seen Alex Hassell in is the TV version of The Miniaturist with Anya Taylor Joy a couple of years back. He played her husband.
There was a discussion on casting a few threads back and he and several others were all dismissed by Cooper enthusiasts because they "don't look like a Greek god!". No one seemed to be able to suggest anyone who supposedly actually does.