I found the tracking number for my passport this morning. It's a good thing I did. When I put it in on the Royal Mail website, it told me they'd tried to deliver it on the 12th! They claimed they'd left a "while you were out" card, but I never saw one :/ AND the 12th was a Saturday morning and I know we were both in.
Anyway, I printed out the page and went to the delivery office. Thank G-d I'd only sent it Special Delivery and not Recorded, or it would've been sent back. New passport now in hand. It's all funky and digitized, unlike my old laminated one.
Now I have to ring Croydon so I can go down there and get my stamp transferred. FLR stamps are transferred for free, but you have to go to the office :/ (ILR stamps are done by post, for a fee.)
So, thumbs up to the US Embassy for efficient service, and thumbs down to our postie.