I haven't gone to the USDA yet. But I have called them and they were pretty informative (the second time). If I remember correctly, I just have to mail them the forms (and the fee) and they mail it back to me certified. I have to double check, but my office was in New York (where I live), so it wouldn't be a big deal to go get myself (just another thing to add to 1000).
They said not to do it until close to your leaving date because it is only good for so many months (2-4ish - I forget). I wish I was more reliable with the details, but the general scheme of it all I'm sure of (now).
My major point of confusion is that I saw NOWHERE else a mention of this USDA approval (on this site or defra). I have since posted as much as I could so people can look into it. But for something so crucial, you'd think there'd be something more clear.
I assure you I will post back once I get over there (Oct), which I assume is before your departure to let you know if the USDA thing was absolutely necessary. Because, believe me, one less step in this whole thing would be an appreciated relief. But as of now, if there's anything remotely required for my dogchild, I'm doing it.
best of luck,
grant