It's also true that this is pretty common practice for intelligence work. Whether you call it "an institutional culture of secrecy" or "sensibly limiting attac

It's also true that this is pretty common practice for intelligence work. Whether you call it "an institutional culture of secrecy" or "sensibly limiting attack surface", there are numerous stories of intelligence projects circumventing allies, other agencies, or even other branches of their same agency. Involving outsiders tends to be viewed as a tradeoff between courtesy plus access and lost secrecy plus bureaucracy. Now, it's also true that intelligence agencies have a habit of grabbing whatever they can once they open a channel. But at least for the initial decision, I agree that the GCHQ would be pretty likely to bypass any other agency they didn't think could substantially help their work or punish the discourtesy. Which would be mostly limited to the NSA and maybe the rest of Five Eyes.