Originally Posted by
mathe
I have US and UK passports. In the past, I've always handled this by showing the airline the US passport (in both directions) and showing immigration officials the passport they expect. However, it seems like the UK now uses advance passenger information, which I don't think was the case the last time I was there. What are the current best practices?
You must not have been to the UK for a long time then.
Simply enter the passport you are showing to the airline in the API (or the airline will do it for you if you don't). In any case, if you are entering the UK with a UK passport, nobody is going to care that a US passport was provided in the API.