Which Lane Upon Exit?

In an ideal world, your assumption would be correct however it will never happen. People will tend to use whatever lane they feel is right for them. I wonder though, why the car park business didn't mark one lane as right only and the other as left only coming out of the exit? I believe that would end any confusion. Are there any signs a little further back into the car park to given an indication as to how each lane should be used or are they just both exits from the covered area?

The above makes the most sense, since then two cars, one going right and one left, can exit simultaneously without getting in each other's way (assuming traffic conditions on the road allow it). After Red turns right, they can take whichever lane they need to join the main road.

The contours of the junction's kerbs are even set up to guide you in these directions, but it seems that they prioritised the No Entry signs over arrows. Which just goes to show how much even grown adults in command of vehicles need explicit instructions or prohibitions about the most simple things.