Sorry, it's the button I the right hand side that's labelled 'Disp'. Press & hold that & it resets whatever is displayed (where possible), MPG, Journey Time, etc
The CT does the same. Pressing P allows you to still use any of the accessories/displays/audio etc as the engine/battery is still running. It also disengages the door locks.
Pressing the On/Off button shuts everything down and disengages the door locks.
The foot brake is really the same as a handbrake on a manually geared or automatic car. Automatic cars still have a handbrake even though you pop the selector into P. If you just use the P selector and not the foot brake it would be like leaving a manual in gear but without the handbrake on.
Hope this helps.