I also went with a pet transport company. Initially I was going to book through Virgin, but I got nervous about the paperwork and with DH relocating and us trying to get rid of all our stuff and me trying to get the visa application together, it's been too crazy. Something could easily be missed, so I went with petrelocation.com and they've been top notch. Their initial cost door-to-door was really exorbitant, but when I cut the door-to-door service and went airport to airport/HARC, that made it seem far more reasonable. Would it have been a little cheaper to do the arrangements myself? Yes, but I liked having the peace of mind and guidance from the transport company.
The transport company walked us through everything, booked her flight on BA, sorted out my vet who had some poor misconceptions of what my dog would need to get through UK customs ("Myths" the transport company called them), FedEx'ed the documents to my vet in advance, was on-call through my final appointment and reviewed all the documents filled out by my vet for any errors (of which there were several!!! We had to do it 3x before they got it right) and provided the overnight FedEx envelope to send to the USDA for their endorsement and the forwarding FedEx envelope to where we dropped our dog off. They also kept us informed every step of the way, providing an update for when she was checked into the flight, providing tracking information so we can see her progress over the Atlantic, and notified us as soon as she cleared customs! It's a huge weight off our shoulders and I'm looking forward to seeing my puppy on Sunday when we arrive in the UK (I'm just visiting until I can apply for the visa upon my return to the US).
In any case, I have to say, the transport company was well worth it for me.