moving cats from US-UK soon! Need some advice..

We are leaving on the 25th of March.  We have 3 cats that are coming with us and we're flying VA.  How do you go about booking something for the cats??  Our first leg from Iowa to Chicago is on United and then our long haul flight is VA.  So we do have to change planes.  Do I contact United or Virgin to book them and how far in advance??  The vet is working on some paperwork and then will help us finish up the last right before flight stuff and make sure we have everything from them.  Anyone with experience or any ideas??  I would start calling myself, but I'm stuck at work and usually I get better info on here anyways!!

From what I remember researching on my own and reading here in the forums, it's imperative to contact them in advance... and then again at intervals leading up to the flights to confirm. 

I believe there are folks that have done flight changes/diff legs as you will be so they can prob provide guidance here but my assumption (and I think it's a safe one) is that you will need to contact BOTH airlines to arrange this. 

Personally, I wouldn't risk it by waiting, I'd contact them asap just to be sure there aren't any kinks, and if there are, you'll still have time to resolve. 

Thanks for the advice!  I was wondering how it works on flights with connections.  There isn't a direct flight from here at all and I'm really worried that the cats will get lost in transit somewhere!  I'll contact both ASAP.  Thanks!