Proof of Concept
Encryption is not the issue. Here's a simple proof of concept that you can identify the content shared by individuals.
Say you want to download the latest Star Trek movie using bit torrent. You search for the torrent, put the details into your bit torrent client and it downloads the content (encrypted or not) from various IP addresses belonging to people across the UK and elsewhere. Eventually you have the file on your machine.
Obviously, the P2P system contains all the information you need to go from identified content.
The only extra steps a surveillance system has to do is verify that the downloaded file is indeed copyright, then repeat the P2P query process to obtain IP addresses.
This isn't necessarily the only way to do this, but it proves it is possible even when the streams are encrypted. Encryption is good, but it isn't magic. TOR on the other hand...