Since the car park does not require payment, (a) there can be no commercial justification, and (b) can there even be a contract to enforce?
I think (a) would be hard to argue on the basis of the car park being free. The car park in ParkingEye vs Beavis was free (from memory), and it was that case in which the Supreme Court ruled on the commercial justification issue in the first place. In a retail park car park, I think it would be easy for the operator to argue there is a commercial justification in ensuring people don't park in a way that takes up 2 bays, as this reduces the number of bays available for genuine customers who might wish to spend money at the retail park.
However, in this case I agree there's an argument to be made around commercial justification, although I think the angle to go at it from is that the end space the OP's car was straddling was unusable anyway, so the driver didn't reduce the capacity of the car park by parking in that manner (which would generally be the commercial justification for enforcing against cars that straddle bays).
On (b), there generally doesn't need to be money changing hands for a contract to be formed, as long as there is some 'consideration'.