This question already has an answer here:
I've read several great posts about traveling with 2 passports, specifically German and US. I'll be leaving Boston for Berlin with a layover in Iceland. In Iceland and Germany I will show my German passport. I know that when I am ready to leave Germany, I need to check in with my US passport and then go through exit customs showing my German passport.
My question is the following:
On my return flight from Munich to Boston, I have a layover in London. Which passport do I show them at customs upon entering? And which passport do I use in exit customs?