I have looked into doing it ourselves (there's only one airline, Delta, which flies direct from Heathrow to Detroit, so it's not too much research!). But after adding up the costs of 2 cats as excess baggage, buying 2 kennels, buying 2 sets of accessories for those kennels to make them comfortable, and paying for 2 vet certificates, it only saves about 150-200 pounds, which to me isn't worth it.
We're going to be so busy with vacating our flat, shipping our stuff back to Michigan and selling/givingn away the rest, and getting ourselves and all the other excess luggage to the airport, that I thought having the cats at the kennel the day before and then dropped off at the airport by someone else (who knows what they're doing!) would relieve some of the stress.
I'm open to suggestions though if anyone knows better, but for me, when it comes to the cats (and only to the cats!) money is not the main issue. I want what will make them most comfortable and most likely to get there safe and sound.