I told the OP my story from this past summer when we brought our cat from the US.
Delta required that the cat be accompanied, and because of the summer heat embargo, we had to originate the flight at a gateway city in the US. For us this meant a 10-hour drive from Richmond, VA to Atlanta.
At that time, Continental would have taken our cat unaccompanied, but again we had to fly her from a gateway city, and that would have been Newark, NJ. Closer to Richmond VA, but the traffic, drive would have been even worse that going to Atlanta.
The airlines have their own rules - Delta required the USDA paperwork, Continental only required the DEFRA vet certificate.
Airlines have rules, airports have rules and DEFRA have rules, and unfortunately, flying to MAN versus LHR or GAT can mean an additional layer. I agree based on my own experience that some agents don't have a clue, and give incorrect information, but you really do have to check at every level to make sure the animal can get on a flight.