Avoid the Milltek they crack. Non-res is not that loud if you still have both cats. Even with the second cat removed it isn't that loud, though will be on cold idle on initial start up. Have a look at the TD systems, they do three options. A regular resonated system is a safe bet with your setup. You may not feel it's loud enough though. It will be fine if you for a manifold primary decat in the future.
Contrary to what @MartinT said a non-res when you still have cats will not drone on the motorway. Even with the second removed it's fine. Cruising is never a problem really, you will hear it when you open the throttle, but that's kind of the point.
I went through four Milltek exhausts before I got sick of the cracking and went for a TD resonated when I put a TD manifold on. I had no cats at this point. It was a little too loud, so I put some resonators (2x extra silencers) where the second cat would be. It's slightly too quiet, but I've got used to it now and am happy enough with it. No more cracks!