I think a lot of the post July 125i owner will like this.
I have a 125i which was delivered in Sep last year. Now have done around 7,000km. I tried the launch control today, and it worked!!
I drove the car for just over 10km (with auto start/stop off). Switch to Sports+ mode. Press the traction button (I looked at the screen and can see the word Sports+ have changed to "traction"). Foot flat on brake, and then flat on gas. Then I can see the flag logo and said "Launch control activated". Just like DECVING said, it actually feel like a clutch