Originally Posted by
swiftaw
So which passport would I use for API? UK one on the outbound flight, and US on the return? Or does it matter?
You are required to leave and re-enter the US on your US passport so you need to use that for API. The airline will check on departure and both the airline and CBP on return. You can use the UK passport for entry to the UK. I haven't yet had Border Force ask why I do not show as having been on an incoming flight and they don't ask for that information whereas CBP know and check.