I think their long term goal is for the majority of service to operate from LHR-USA, so I think this is more of a move for open skies, or at least putting pressure to allow them to have LHR service.
However, MAN-IAD will do very well in the summer. Most Transatlantic flights from IAD in the summer are sold to capacity or oversold to the max, day after day. MAN-ORD would also be a great flight when they start it, especially when code-sharing with United.
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