Presumably Gatwick is the same as Heathrow - if you are connecting then they record your boarding pass, and take a picture of you, at flight connections.
Correct, that's how they do it in mixed departure halls with airside transit; this would prevent you doing the boarding card switch because your photograph would not be associated with the domestic boarding card.
There might, however, be a slight vulnerability in that airline staff occasionally just override if a boarding card won't scan, but because you wouldn't be able to control if that happened or not it would not be a useful vulnerability to exploit deliberately.
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As has been pointed out, LHR certainly (don't know about LGW) now takes a picture of all departing and connecting passengers which significantly minimises the risk of a boarding pass being handed over to someone else. However, EDI (which has far fewer connections than Heathrow in all fairness) has no such system in place.
EDI doesn't (in common with most smaller UK airports) have a flight connections facility. You have to enter the UK and repass security to connect. You only end up getting off a flight straight into departures if it originated from the UK (because UK security, and only UK security, is trusted). Therefore only people who have legally entered the UK are in departures.
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Not sure if this is still possible. I hope not.
The CTA is an odd vulnerability in a lot of places. I do think we should move towards either a full Schengen-style open-borders scheme with Ireland with shared visas etc (in which case these would be domestic flights), or just abolish it and have a proper border.
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No, this is not possible. International flights in the UK all clear immigration on arrival, and this includes all connecting passengers.
No, they don't. At airports with an airside transit facility (in the UK this is probably just LHR and LGW, I doubt any others are big enough though Manchester might have it) you do not have to enter the UK if you are going to depart on another international flight.
Last time I went to America, I landed at LHR T3 and had to pass through immigration and security at Terminal 2 before I could catch my domestic flight back home.
This is true of domestics, though, as having to split a domestic between "has entered the UK" and "has not entered the UK" would be complicated.
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Stansted and at least one Gatwick terminal used to land Irish flights at international gates rather than domestic ones, but with a lane to follow at passport control which just checked boarding passes rather than passports.
So I did worry at one time that you could:
If they were suspicious they could check if you boarded the Irish flight, I suppose, but that depends on them being suspicious.