So plotting to kill someone (thoughts) with evidence supporting those thoughts is not a crime because no physical act was committed?
No, it's not, unless that evidence is extremely convincing.
As an example, there was a bank down the road from me that got robbed was I was about 15 (they got caught within 20 minutes). Me and a few mates got into a discussion on how we would rob the bank.
It turned into a game, and we made all sorts of detailed plans on how to hit the bank and escape on dirt bikes (with multiple routes) We talked about this for years, had plans drawn up of the bank and our escape routes. Of course it was just a fantasy - we never had any intention of robbing the bank. People think (and even talk) about doing a lot of things, that doesn't make them guilty of conspiracy. In hindsight, it would have looked bad if the police had ever found our "plans", but it was just an amusing way to pass time, nothing more.
Conspiracy is very difficult to prove, as you must also prove true intent to commit a crime. Photos of a building, architectural plans, plus a load of Semtex and detonators. Now that would pass as true intent.
