Originally Posted by
Qwerty112345
Thanks all, very helpful responses. Sounds as though on the basis that you have to pass immigration you probably will also need to go through customs which means you have access to your bag and can quietly leave the remaining passengers at AGP. Although not a thesis which I would ever want to test.
As for not having the Schengen visa - my understanding from other threads is that people have been refused entry to Spain and forced to fly back to the UK in this circumstance. Ouch!
I didn't want to get into it too much but if you're somehow worried about customs, you will have to go through customs to get into Gib and then once again when getting back into Spain as Gibraltar is not in the customs union (despite being EU....lots of asterisks apply to Gibraltar). If you fly to AGP, then you would not go through customs since UK is currently in the customs union.