Weasels
Ok, the facts as I understand it:
BT have trialled Phorm on part of their network which has intercepted the telecommunications of some of its customers without their consent. This is illegal under RIPA and BT have commited an illegal act as a result.
However, because the interception was done without determining the identities of the customers involved (and Phorm does not record this information anywhere) it is not possible for BT to determine who was a victim of this illegal act. It is therefore impossible to raise charges against BT because the victims cannot be identified. BT know this.
I think that the only way that it is possible to charge BT would be if someone can prove that they were part of the trial, and therefore that their traffic was intercepted. Something which BT was hoping wouldn't happen because no-one was supposed to find out!