You don't need an agent on the US side, but Delta has an arrangement with Pets on Jets for Manchester on the UK side. Continental also have a specific arrangement, but can't remember the name of the firm.
POJ checked papers that I emailed as a PDF file prior to the flight, and were quick to answer questions. The charge was £130 - far less than the $730 a Delta cargo employee had quoted (this was in July this year).
I've always wondered if the Delta agent misread the "1" as a "7".
We delivered our cat to the Delta cargo terminal in Atlanta, paid the cargo fee, and that was that. Animals flying into MAN must be accompanied - perhaps the higher fee is for flights into LHR? Not a clue on that.
Be aware that OJ ion Manchester do NOT take a check. It's cash or (£) or credit card.