If you still hold a UK passport then enter the UK on that passport and as far as the UK is concerned, you are a UK citizen. When you re-enter the US, enter on your US passport. because as far as the US is concerned you are a US citizen. If you only hold a US passport, then put "US Citizen".
It is perfectly acceptable to carry two passports and enter different countries with each. People do it all the time.
helwardman got a reaction from Alex & Rachel in Traveling to UK after US Citizenship