They check the sound, but not emissions... unless they can smell you're running a decat or you're running extra extra rich.
But saying that, they can just tell you to get a emissions check, though... but this doesn't happen all too often. Didn't happen to mine. Just make sure your exhaust is pretty quiet.
Yes. Trimmed number plates are a big no no. BUT it depends on just how bent/trimmed it is. If it's slightly cut, I've had many mates get through with it. But if it's hacked up or bent badly, they'll fail you.