e.g., if you take the EURO star, you show you Schegen visa to the French control and they will stamp it says you leave Schegen region
then 5 metres behind (notice for EUROSTAR, UK Border Agency is in PARIS!!!) you will see the UK Border Agency. Then you show your UK visa and fill the landing card as well. Well, if they ask you for details, you can explain.
the border control are in different places
e.g., if you take the EURO star, you show you Schegen visa to the French control and they will stamp it says you leave Schegen region
then 5 metres behind (notice for EUROSTAR, UK Border Agency is in PARIS!!!) you will see the UK Border Agency. Then you show your UK visa and fill the landing card as well. Well, if they ask you for details, you can explain.