Great news indeed (though they announced that this would happen when they applied for the joint venture with VS, of course). The idea is that that marketing bonus will make this stick when the NW service couldn't.

With 4 hours in SEA, I'd assume one aircraft is doing LHR-SEA-LHR (at least normally), so it will obviously cycle somewhere else at the LHR end. Plenty of 763s cycle through both airports to give them some flexibility, though.