Do you have an MG4? (Your profile says otherwise - advice may differ depending on what exact car you have).
In the MG4 you need to go to the main screen, select the Car icon, select the MG Pilot tab and then select Cruise Control Mode (I think) rather than Speed Limit Mode. Once selected then you have a choice of Off, ACC or TJA. Assuming ACC or TJA is selected, then pressing the middle left button on the steering wheel (looks like a steering wheel) will enable ACC (or TJA) at the current speed. A tap up of the left joystick will set the speed to the nearest 5 mph increment - tap down will set it to the nearest 5 mph decrement.
If you drop out of ACC (e.g. you pressed the brake pedal) then a tap up of the joystick will resume ACC at the previously chosen speed. Pressing the middle left button will resume at the current road speed.