Years ago, Detroit was often mooted but never happened. I thought the financial crisis had taken a heavy toll on industry there - are there signs of recovery?
Seattle doesn't really fit the connections criteria (I assume they would be at the other end not MAN) and the wrong industries I would have thought for cargo.
Pittsburg, an industrial city, was once a possibility but as far as I know has never developed as an international airport.
Phoenix or Denver come to mind for possible connections, but not sure as to whether they would be a driver for cargo, and Denver is a place you might want to visit so doesn't fit with the hints.
If, and it's not been confirmed, it is a US route, we seem to be left with BA and DL (UA, VS & AA, which I take to include US Air now, having been ruled out). If it's not the US, and not a transit flight, and it's a current daily operator into MAN, the choice is somewhat restricted I would have thought.
I bet LAX LHR is enjoying this!