I have an older Leaf that uses an electric parking brake (Nissan went to foot-operated for 2013). If it's on level ground, I don't necessarily use it, but if the car is on a slope, you betcha.
I'm don't how the Park position works on the Leaf, but on an ICEV it's little more than a relatively small piece of metal that hooks itself into the teeth of one of the gears in the transmission. That's all that stops your unattended car from causing death and destruction, unless you use a backup method like the parking brake or putting chocks under the wheels. Some companies especially those that drive large trucks make their drivers chock the wheels anytime the vehicle is parked.
One member here posted how she inadvertently left her car in the "Run" position and then plugged an EVSE into it. Most if not all of us were not aware of this behavior at the time, but apparently if you do this while the car is in "Run" the car automatically switches to Neutral. The parking brake was not set so the car rolled backward and eventually hit a tree. Fortunately it didn't hit anybody else as a result, but the owner was injured in her failed attempt to stop the car from rolling away.
As you are in the UK, the Highway Code (section 239) says you MUST apply the Parking Brake before leaving the car.
