Originally Posted by
flatlander
It seems like the UK has been doing this for air passengers leaving the country for a while. So they must have been dealing with the question of someone arriving on one passport and departing on another in some way - possibly by ignoring it.
If we assume that everyone is checked on the way in across all border crossings (titter ye not!) then foreigners leaving who did not arrive, are all going to be people leaving on the foreign passport after arriving on the UK passport. The government need only concern itself with foreigners who arrive and then leave after they should. Simples!
Using advance passenger information (API) from the airlines they could get consistent arrival/departure data for simple round trips even if the API doesn't match the passport used at the border control point. (Though the PNR could list both passports, if the airline collected that info.)
When I flew from Boston to London and back last year I gave VS only my US passport info, but when I arrived at LHR I left that passport in my pocket and entered through the e-passport lanes with my IE passport. API would match up my arrival and departure flights, though if they didn't correlate my actual entry with the API (which could be done fairly easily using my name and birthdate, which are the same on both passports) then Border Force might assume I spent two weeks sitting in the transit lounge at LHR T3.
