RE: being new

I’m not sure. Certainly the GLA JFK flight was a 757, so I assume that was the long haul configured 75. However, BA no longer operate that flight. I thought it (the plane) may have possibly been moved to Manchester for long haul flights from there, but I don’t think it was.
Incidentally, the shuttle flights between LHR and GLA/EDI/MAN are not all economy. They’re shuttle class, which is booking class C.