Don't feel too bad, it is not just you. I regularly travel on positioning flights with BA including flights from Manchester. Always found them to be excellent. However just after they subbed out their check-in to Aviance, I arrived early one morning with no queue at all, and well in advance of my flight departure time. Depending on the fare paid for my ticket there sometimes is and sometimes isn't lounge access, so I always quietly ask, as I usually just have a locator code and a departure time. On this one occaission I was a little surprised to have my polite question retorted with a firm no! followed by much typing. After passing through security and buying a newspaper, I sat myself down and telephoned home. My wife made some enquiries of her own and phoned back to say that I could use the lounge, but the supervisor was amused to note the comments the check-in agent had typed, suggesting that I would attempt to access and must be refused!
I think every company has a frustrated wannabee who never quite made the grade, and feels taking it out on the rest of the world is their just compensation. Clearly Aviance at Manchester has this one. It is like those ones you see every week on those "airline" type docusoaps who never seem to notice just how dim and ignorant they make themselves look, as long as they get to see their own face on TV.
You are right the service isn't BA, however it is a sub-contractor of BA who are doing their customer no service, and their customers customers an even worse one. To be fair to Aviance, I suspect this type of behaviour isn't typical, and probably makes others who work for them cringe as well.