Miranda was flying from Berlin to Rio de Janeiro with a change of planes in London.
Since you pretty much can't fly under an alias these days, it was probably pretty easy to identify him as he arrived from Berlin. The UK knew the exact seat he was sitting in.
Yes, but first someone in the British government had to have put this man on a list. Some government official consciously come up with a list of names of people to go after.
It probably wasn't a UK list, might well just be a US one, but it's interesting that we have no idea what lists there are of people of interest, who makes them, or how people end up on the lists or are taken off them (if they ever are). See Laura Poitras being stopped and searched in Sarajevo based on a US watch list for example:
Since you pretty much can't fly under an alias these days, it was probably pretty easy to identify him as he arrived from Berlin. The UK knew the exact seat he was sitting in.