How would they have gotten this flag?
How? I don't think you understand exactly what you're suggesting (or perhaps I don't). If the prepping takes place in another country (which it does) how do our security services "take them down" and before we start harping on about special forces, they aren't some omnipresent crack team.
So the next plausible suggestion is a large occupying force in these places where the planning tales place... We know how well that went down last time.
Time to arm the UK police
No it isn't
No it isn't
Yes it is.
No it isn't
Thankfully the police don't agree with you