Simon AKA 'Madone' on here got a box rebuilt by a guy in Oxfordshire to a better spec than standard. I'm not too sure about how much it cost him, but bung him a pm and he'll be able to tell you.
As for Dubshack, they do some very good work, so I think you'll be alright there, but I've never heard of Race Power Motorsport.
Another couple of places you might want to try is Anderson Race Engines near Bath, and Automotive Solutions just outside Reading.