Originally Posted by
simpleSnow
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.