On the chance that this is true (I have no reason to trust the British government on any surveillance topic), that would mean one of three things: 1. Snowden messed up and chose a bad encryption scheme/key. 2. Russia and China have the ability to break secure encryption schemes. 3. One of the journalists entrusted with the documents messed up with the handling of the keys.

Only number 3 seems a little bit possible, and even then I'm skeptical...