As someone who lives in the UK, despite the National Crime Agency (NCA) having been dubbed by the media as our version of the FBI, the truth is that due to the young nature of the agency at the moment, they are not. Nation-wide, counter terrorism is still handeled by the security services (MI5, MI6, GCHQ and the Met police's SO15 Counter Terrorism Command). As for gun smuggling while the remit of the NCA, most opperations are carried out by either UK Border Agency (UKBA) and/or regional police forces with support from the NCA.
Thanks for your comment. I had I heard similar things.