That excerpt from the highway code is all I could find too but it doesn't define what it means by "parking lights". It was some time ago when I took my test, but I remembered parking lights being the side lights i.e. on both sides of the car. My dad helped me out with this one, he's been driving for 53 years and spent many of those years as a driving instructor. Turns out the minimum is to have the offside illuminated. You can't park against the flow of traffic, so that means the side furthest from the kerb. Both sides is fine, hence why most cars don't offer the option of one side without the other.