UK API System for Separate Trips
 

Will this be an issue with the UK API System?

Flying roundtrip to UK from US on US passport, so I enter the UK on US passport. However, buying a roundtrip ticket from UK to EU using my EU passport. Then after the EU trip I return back to UK then back to the US on my US passport.

But with this won't the API system see me leaving UK on EU passport but I never entered UK with that passport? Won't that be an issue? Then if I re-enter from EU to UK with US passport it will show I entered two times which could raise an issue.

What's the best way to deal with this API System on separate trips / two passports? Thanks.

To enter and exit a(ny) country with the same passport?

I understand that, but with UK's system there are no exit controls, so I assume if I checking in with a given passport it's counted that I exited with that passport? hence the other passport could be flagged as having overstayed?

Or let's say the following ; I enter UK with passport A ; I leave UK with passport B ; then I return to the UK and enter with passport A and finally leave with passport A.

So the issue here will be that I entered UK with passport A a second time without leaving?

No, its not an issue. In the unlikely circumstance that passport control actually catches this AND cared enough to ask, you would simply show them the other passport and while it might take a few minutes, they will reconcile actual entries/exits. One reason to keep old passports and boarding passes. Also why exit stamps are (were) useful.