Originally Posted by
Aviatrix
As has already been pointed out, someone could have a UK visa but no Schengen visa.
However... it is very unlikely that anybody would have been able to board that flight unless they had the right documentation to enter Schengen. They would have been turned away at check-in or at the gate.
And I can confirm that there are no airside transfers at SOU - though that wouldn't have made a difference as passengers would still have had their passports checked at the gate.
They wouldn't have been turned away at check-in though if they OLCI'd or used SSCI and no baggage to check-in. And do the GA's really check passports properly anyway? - they only look at the photo-ID page and not whether the passport has the correct stamps or visa entries IME.