I am not sure it's outright illegal to have auto start. One of those grey area things like HIDs. You can install aftermarket auto start which I don't think is against the law.
Going back just over 10 years, I knew someone who installed auto start on their car. An old 80s Fiesta of all cars and I had no idea this was even possible. Long story short, the guy showed off his new toy, got in his running car and forgot something. He left the key in the car and leapt out for a few seconds but in shutting the door, it locked. So he was stuck with a chavvy noisy Fiesta (it had all sorts of Halfords crap on it which I believe is where he purchased the auto start) just running on the yard at work. He never did it again, as a workforce of 50 odd men found it all too amusing!
Personally, I'd be warier of the new breed of ultra dictatorial council who are introducing byelaws for just about everything. Some, particularly in London, will fine you heavily for having a running car at the side of the road for more than a few seconds.