<human>: Can you provide a brief overview of the text?

I'm a dual citizen (US and Germany- just got my German citizenship a few months ago) I'm going to England for spring break and am planning on using my German passport because I don't want to wait 1+ hours in the non EU/EEA immigration line.  When I got to the traveller info on the Virgin Atlantic site, it asks for passport information but its unclear if its just for the return flight to the US, and if you want to enter a non US passport it asks if you've applied for an ESTA (which I haven't). So which passport do I enter-- German or US?
<bot>: Going to UK as a dual (US and Germany). Which passport do I enter on the airline site or show the airline upon check in?