As Si & BCM it's illegal to leave your car running unattended.
Plus the fact that if you have to wire it into the alarm to get round the Immoboliser the it will invalidate you CAT 1 alarm then TBH they're just not worth the hassle.
As has been said, just take the last bit of you journey nice an easy, and then let your car sit for a minute or 2 on the drive before you switch off.
I've gotten into the habbit of listening to the next song on the radio once I've parked up before I switch off, which should be enough time (as long as you haven't ragged the hell out of it up to that point).