A quick expedia search for New York and Boston flights during the summer
months reveals that EI are indeed very competitive at BHX and BRS, whilst not so much at MAN
The transfer market for BRS/BHX will be quite good for the SNN routes as those 2 airports do not have much Trans-Atlantic offering (none at BRS and just 1 daily United to Newark for BHX).
The MAN issue is that for the SNN route, the only selling points are BOS and the pre-clearance, as MAN has quite a large Trans-Atlantic offering.