Not really, the police can't just phone up the ISP and ask for an IP address, that IP address is protected by law, a warrant is required or the ISP is required to breach the law, something a lawyer would then use as part of your defence.
Put it this way, you're as safe or as unsafe as you want to feel. You can live being paranoid that the helicopter is up there for you, or you can ignore it smoke a joint and smile when it flies on by and nothing happens. You only live once, no point wasting your life being ultra paranoid. There are very few uk members on this forum who have anything going that would constitute a real punishment, and the ones that do, well they don't post them, americans seem to be more willing to post about their big grow ops.
If i were being paranoid, i would say that joining a forum with an initial posts asking a question on whether police use this site, and then following up with a post stating it would be a no-brainer, my paranoia would claim you're more than a simple question asker