I flew to Gatwick, but I used Continental (who were fab). Once the pet is at the ARC, it's out of the hands of the airline. Once the cats arrived into Gatwick they cleaned out the one crate because my cat peed in it, bagging up the towels and such that I had in the crate. The Gatwick ARC is not directly attached to the airport, so you will need to take your rental car or get a taxi out to the center. My cats were definitely not at the ARC for 4 hours, which is probably due to a small number of animals coming in at the same time. The people at the ARC were very friendly and I had no problems with them.
My advice is to do less research on the ARC and more on how Delta handles pets. I'm sure they'll be fine!