Hello fellow Seattleite
Here is the link to the Defra website, they are the org that manages the PETS scheme.
http://www.defra.gov.uk/animalh/quarantine/pets/index.htmBA is very likely the only approved route from Seattle and honestly, with everything I've been reading on these forums about other folks' experience and what I've researched in addition myself, it's really the only way to go. It's non-stop and I hear the BA cargo folks are some of the best when it comes to handling pets and putting pet owners at ease.
The PETS process is a bit daunting and if you search around you'll hear folks advise to start approx 8 months out in order to get everything done and also comply with the 6 month waiting period after the bloodwork, etc. Lots of folks have done it though, give the Defra site a good read and then post some specific questions. I'm sure folks that have actually gone through the process can help summarize the steps.
I'm contemplating the move to UK with my pets from Seattle so I'm always searching for info on folks having done so from the West coast as it's a much longer trip.
Good luck!