A government CA wouldn't give them anything they want, let alone anything they don't already have. Even if we're naive enough to assume GCHQ don't already have

A government CA wouldn't give them anything they want, let alone anything they don't already have. Even if we're naive enough to assume GCHQ don't already have the private keys of half a dozen root CAs, owning a CA just doesn't enable cheap, post-facto, mass surveillance.