I’m in the same boat. We’re due to fly on 14 December. Got my UK passport renewed, but now I need my US passport back. I called the passport office today, and they said it could take up to two months from when they’ve issued the new passport. They have no way of tracking where the supporting documents are or how when they’ll be sent back.
I really don’t want to have to apply for an emergency passport, but cancelling the trip is not an option.