Turner is a villain, someone on par with Bond.
A bit like Trevalyan to Bond. That's how I see him.
Josh O'Connor is more a Mr Moneypenny.
My choice is on Lowden, not only because I'd like to see him in a proper action role, but also for exactly the reason you have with Slough House.
Honestly, I think I would have preferred a Black Bond, or maybe even Riz Ahmed, but it's slim pickings now Elba and Ejiofor have aged out of the role.