Malta and Gibraltar obviously played themselves, as presumably did
Brussels in "Enough of Ghosts".
I'd guess the Russian scenes in "First Principles" were shot in Scotland.
The border at the beginning of "Is Your Journey Really Necessary" could
be Wales or the Lake Country.
Istanbul, I'd guess, was represented by stock shots; there wasn't any
exterior action in "Decision by Committee". In "At All Costs", if memory
serves, there were no long shots of Sofia, so those scenes could be anywhere.
I can't remember whether there were any exteriors of Prague in "A Proper
Function of Government" (...or whichever episode took Willie there; my
notes are somewhat damaged).
But in "A Question of Loyalty", we see some wide shots of Mike in Warsaw;
where was he? (And in the same episode, where is the bridge on which Karen
gave Neil a clue? I never visited that part of London.)
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