They monitor people who are already known, and for those that aren't, they look for stuff that's a bit more obvious than hiring a van and driving to a different city. Otherwise you'd have armed police swooping in every time someone bought a sofa off eBay.
So Twitter following Britain First isn't sufficient to raise an eyebrow from those tasked with protecting us?
I'm not suggesting hiring a van is of itself sufficient, it is Darren Osborne's combination of actions that should have resulted in him being pulled over.
So what does GCHQ do 24/7?