I flew from Dulles to Heathrow on Virgin. It cost about $350 per cat (I have two). I'm afraid that unless they're kittens, I think UK import regulations state that they have to go in seperate carriers. And the carriers have to be bigger than you might think: two inches higher than the standing cat's ears. We bought a pair, and then had to go get another set of carriers when the ones we had turned out to be too small (the shipping people should be able to check your pet in the carrier the day before the flight to give you time to fix things! they did for us. And definitely take advantage of this: there was another couple flying with a cat on our flight, and they hadn't checked--their carrier was too small. The man went out to buy a carrier, but I guess he didn't suceed because I saw them at the airport, but they didn't come to get a cat.). Another thing to know is that it costs £220 *cash* to retrieve the animals at Heathrow (they might take a check on a UK bank, I don't remember, but at the time we didn't have a UK bank). They don't take travellers checks or credit cards. I brought travellers checks and cashed them in at the airport. And one more thing: the taxi from the airport to the animal facility cost me £20, which was more than I had expected.
Good luck with your travels!