IANAL

But how does one go about starting a private prosecution?

Presumably, anyone caught with a spliff can now claim that they took advice from their dealer, and cannabis has now been ruled legal in the UK - a genuine misunderstanding guv, no criminality meant, probably won't do it again.

The ICO is always complaining he doesn't have powers to go after or punish data losses - well here is a clear case for him to prosecute to clearly lay down a marker - content transmitted or received by an ISP on a customers behalf is both private and sacrosanct. You must not look at the content, other than to maintain the efficient running of your network. Trying to profit from your customers data is wrong and immoral, and when caught you will be seriously punished.

I think a massive fine is in order in this case, with directors going to jail if it happens again. Proper jail too, not Ford Country Club.

The whole thing is incredulous. From BT and their lawyers thinking it was legal in the first place, to CPS thinking there is no merit in prosecuting, or that they won't get a conviction when BT have admitted to doing it. I can imagine how the conversation went at CPS:

Lawyer A: So they admitted to it?

Lawyer B: Yep, no chance of a conviction now.

Lawyer A: Well, they didn't mean to, ignorance of the law is a valid defence, right?

Lawyer B: Well, I'm ignorant of the law, so yeah, guess so.