Moving back to UK from USA and Heathcare
#46

I am a dual citizen UK/ USA. I moved back to the UK in December 2018 but I am going to the USA for 3 weeks in May. My question is when I am booking my flight to the US which passport number do I use? Also when I get to the airport which passport do I use when I leave the UK? ….
Under US law you are required to enter the US using your US passport, and if you try to depart the UK using your UK passport the airline will require you to have an ESTA (otherwise they won't let you board the aircraft) which you aren't eligible for if you have a US passport, so you pretty much have to use your US passport for the flight if you are a dual US/UK citizen.
You can use either of your passports when entering the UK, though it might lead to an odd record in the UK passport system if you are a British citizen living in the UK and enter on your US passport, because it would show you as an overstay after six months.
….. Also on the way back which passport do I use to check in at the US airport? I know that once I get to the UK I go through immigration with my British passport....

#47

Under US law you are required to enter the US using your US passport, and if you try to depart the UK using your UK passport the airline will require you to have an ESTA (otherwise they won't let you board the aircraft) which you aren't eligible for if you have a US passport, so you pretty much have to use your US passport for the flight if you are a dual US/UK citizen.
You can use either of your passports when entering the UK, though it might lead to an odd record in the UK passport system if you are a British citizen living in the UK and enter on your US passport, because it would show you as an overstay after six months.