I wish, as then there'd be hope for a smoother and less tailgaty ACC for me.

I used it today, and overall it was fairly ok, but then someone moved on to my lane, behind the car I was following, and sure enough after a delay of 1-2 seconds the XK braked rather hard. The it accelerated again, then braked a bit, then levelled out. After that I switched it off because I'm honestly a bit worried about it surprising someone behind me.

Mind you, mine is a very early example; the VIN is B00031 while the parts catalogue and service manual start at B00379! It's apparently a pre-production press car, so who knows, I might be running on beta software here. My car certainly does have a few minor oddities vs. other XKs.
The production cars, not to mention later ones, might have a smoother ACC.

If I were to hazard a guess; differing legislation and degree of autonomy/automatic stopping or such?

 

I love the ACC in my 5.0, very smooth acceleration & braking however I have never been a guy that uses cruise control much. I just occasionally use it if i'm feeling a little lazy. I will test mine out & report if it brings the car to a complete stop. I'll try & do it today & post the results tomorrow.

 

I had to take over the braking from I guess 18MPH & down to zero so not coming to a full stop on its own.

 

I am with Cee Jay a thousand percent on this. I love my ACC, use it frequently here in LA-- always set it to the closest setting, and always wish I was just a bit closer to the car in front of me. And yeah people cut in front of me sometimes because of that gap. Otherwise its great.

Also, for the OP: I'm sure this has been mentioned elsewhere, but 2010 is the only year that ACC was standard. It was never a popular option and it's rare to find on other years.