Peer 2 peer is actually not illegal. It's only illegal when you're sharing copyrighted material... unless you're the owner of that material. It's should be perfectly ok if you share your holiday pictures or your selfmade music.
I don't think they could read out your IP-address and prosecute you just because you're using P2P software. They would also need evidence that you're sharing copyrighted material. I think that could only be done if they physically seize your computer, before you can remove the illegal material.
Given the amount of people using this kind of software, it's very unlikely you'll be picked out to be prosecuted. It would be like winning the lottery.