Don’t know if what Napolitano said is true or not, but what do you expect the Brit intel organization to say, yea we did it. They are no more apt to say they did it than the FBI, DOJ, CIA or NSA would be. There are huge ramifications for any of them to admit to it if they did it.
GCHQ isn’t admitting to anything. They are on the phone screaming at their counterparts across the pond saying you better make this stop or we’ll blow one of your ops over here.
You can be sure there’s been plenty of mutual back rubbing.