it's a (2) from me.
just a bit of background for any that aren't aware.
(1) (press the P button) is effectively putting the car into Park mode. Handy if you want to be sure that catching the accelerator won't move the car, but probably overkill otherwise.
(2) is the EPB (that's the Electronic Parking Brake). It can be used in an emergency brake, but that's not it's primary function.
(3) That's Auto-Hold. The idea is that you just switch it on and let it do it's thing. Basically, it gets rid of 'creep' once you have stopped. It uses hydraulics (or something) to hold the brake pedal down for you once you have stopped. When you press the accelerator it'll just let go and set off, same as (2). No need to turn (a) on and off as you're driving. Also, if you're stopped for too long (not sure if that's 30s or a minute) it then puts the EPB on and lets the brake pedal go, so effectively you're back at (2) anyway.
Things may have changed since I took my driving test, but I was taught to apply the parking brake and let off the foot brake when at lights or stopped in traffic. That has stayed with me regardless of electric or hand operated parking brake. As you say though, with electric ones, you don't need to find the bite point with the clutch (if you have one...) and you just go. It releases automatically. As a side note though, the switch is a bit too far back and less accessible on the MG5 than it has been on any of my previous cars.
The only thing that stops me using 3 is that it's not particularly intelligent with the brake lights. I don't like dazzling the driver behind but whilst auto hold has it held on the foot brake the lights stay on.