Can anyone please teach me how to use this function?

Use this function by pressing the button you have highlighted. The centre display of the instrument cluster and the HUD will then display the speed limit icon with the words LIMIT OFF to the left and three dashes above km/h or mph on the right. This indicates the system is enabled but not set.

To set the speed accelerate to the desired speed limit and then push the toggle selector switch (above the speed limit button) down to set the speed to which you wish to limit the vehicle. The central display and HUD should now show the speed limit icon with the word LIMIT to the left and the set speed limit shown to the right.

To adjust the speed setting press the toggle switch up to increase the speed or down to decrease the speed. Press the CANCEL button to turn off the speed limit function but keep the current speed limit settings. Push the toggle switch up to resume the set speed limit.

Pressing the SPEED LIMIT button once a setting has been made will turn the function off and clear the set speed from system memory.

In operation the speed limit icon will change from a green LIMIT to a red OVER LIMIT warning and will flash if the speed of the vehicle exceeds the set limit by 3 km/h (or equivalent MPH). If the speed exceeds this limit for more than 5 seconds then a beep will sound and the display continues to flash.

From my tests of the system, you will feel a resistance on the accelerator pedal when you are near or over the limit set by the function. If you take your foot off the pedal the car will slow. It does not keep the set speed like cruise control. Adaptive Cruise Control in higher spec models, in my experience, will accelerate and brake to maintain speed.

If you need to accelerate in an emergency situation simply press the pedal to the floor to activate the change down detent and you can accelerate as required though all the warnings will display. Once you take your foot off the pedal the car will slow and the set limit will kick in allowing you to get the feedback again.

Chapter 6 of the Owners Manual (starting at 6-54 in the Australian manual) describes the function and its use. The UK manual should have a similar page set.