If what you are saying is the whole story, I would get the best lawyer you can find and go to crown court and plead not guilty if it actually gets that far.
It would take a very strange jury or an extremely crap lawyer for you to be found guilty. The jury would listen to your story including the blackmail attempt see the picture of a 17 yo in underwear and come to the correct conclusion. Presumably the girl would be called to explain her actions as well.
I believe that the police are using scare tactics to get an easy guilty plea and a simple conviction. They / CPS will fold if you stand up to them.
This of course assumes that you have told us the full story.