If they did have them, they'd most likely have to hide their existence for quite some time.
During WWII, after the Allies had broken the Enigma cipher and were reading all the German communication, they still had to allow enemy actions that would result in loss of human life, just so they wouldn't tip their hand and reveal that they had broken the code.
I'd assume it would be the same today - a plausible non-crypto way of getting data came out ("We beat him with a pipe until he told us the password", ala http://xkcd.com/538/ ) would be a better explanation than if they broke a code with some quantum ubercomputer.
GCHQ had public key cryptography in the early 1970s, a few years before Diffie-Helman and RSA did their work. While I think it'd be difficult to hide properly functional quantum computing (given the amount of interaction between public and private sector) it wouldn't surprise me if fine minds on both sides of the atlantic were doing advanced research in the field.
During WWII, after the Allies had broken the Enigma cipher and were reading all the German communication, they still had to allow enemy actions that would result in loss of human life, just so they wouldn't tip their hand and reveal that they had broken the code.
I'd assume it would be the same today - a plausible non-crypto way of getting data came out ("We beat him with a pipe until he told us the password", ala http://xkcd.com/538/ ) would be a better explanation than if they broke a code with some quantum ubercomputer.