"Armed insurrection"? If you think that's on my wish-list, then you haven't paid attention at all.
Anyway, the DHS has been posted at the London airports for years . I am sure they were your Customs & Borders people, not Secret Service. The guy's wife is American national and he is a Brit. It was clearly targetted, and not random. They asked for his social media.
That being said, Barry, I think you should believe whatever you want if that helps you sleep better at night.