I'm such an idiot. I just realised, when looking at a copy I made of my new US passport, that I forgot to sign it. My fiance just sent off our application to Sheffield today.
Should he call the UKVI on Monday? Ask for them to send it back to me?
It just occurred to me that when I went to get my biometrics done, a couple officers looked at it and never said a word. It's just one of those things that's easy to miss! Hopefully won't be too much of an issue.