Sorry if this has already been answered but I don't really have time to read through the 8 pages of this thread.
I'm a customs analyst for FedEx. I deal with this stuff 40 hours a week, just mining paperwork for those vital bits of info and then summarizing it in a computerized document to make life easier for anal official government U.S. Customs Officers.
Heck, if it weren't a temporary export (which my position doesn't handle) there's like a 10% chance that I'd end up processing your shipment if you shipped it FedEx, because my shift deals with most of the UK freight. I even have a couple geeky co-workers who have been following progress on the Pandora as well. Sigh.
Anyway, there are a few key phrases you'll want to include on your invoice that us analysts are trained to zone in on. It looked like they've been covered already in this thread, but I would suggest anyway including all of the following statement on your invoice close to the description of your product.
"Temporary export to the U.S. for assembly. To be subsequently re-exported to the UK for further processing."
Now my job is extremely mind-numbing. The majority of us are almost completely brain-dead most of the day. On top of that, many of the people I work with are frankly just plain stupid. So it wouldn't hurt at all to repeat key sections of that phrase liberally throughout your documents.
One other tip that might help is to include an FCC form, even if the items you're shipping don't require it. I *think*, though I may be wrong, that box 5 on the standard FCC form signifies a temporary export, and that is almost impossible for us to miss.
If you still need help, feel free to e-mail me at SalmonGod@gmail.com - I have access to all the official stuff and know a lot of the stupid customs politics, so I could even get the proper tariff # for you and some document templates and such if that would help.
**EDIT**
A couple things to add after reading through the thread... I shouldn't have, but I couldnt help myself
MacGuyver - "I'm not sure if UPS works the same way, but with Fedex you are able to assign your own customs broker to your account."
We call these CPA accounts and I've always wondered if they're even worth it. Our CPA department seems to be experts at dodging their workload. We'll have explicit documentation stating that a shipment is to be processed by CPA, but they'll invent BS beaurocratic excuses to delegate them to standard processing anyway. I've noticed a couple major corporations drop their names from the CPA list recently, too.
Regardless, I would still recommend FedEx. Not because I work there or I think we're more dedicated or anything, but because FedEx brokerage works directly with U.S. Customs officers. We literally share offices. If there are problems, we have people that will just walk down the hall and talk with them, though they'll avoid it if they can because they're anything but friendly people. Because of our direct relationship, we kind of get to work under special rules and get more privileges then other brokers.