While it may have been established that MAN-EWR is unlikely to be cut, I would be interested to see why you think a 2 weekly Icelandair would be perceived as such a threat to UA at BHX, but, in the case of MAN, a 3 weekly Icelandair, 7 weekly AA to JFK and all the other USA carriers would not be seen as a bigger and more direct threat?
The competition at MAN is already there and has been for a long time; United have the BHX-USA direct market to themselves and I'm sure FI will be chasing a lot of this traffic and UA won't be willing to just hand it to them. For all we all know BHX could also have been under threat also but fought harder to keep there only transatlantic service.