From the Traffic Signs Manual Chapter 3
7.29
The sign to diagram 660.4 (the loading only sign) must be used with a bay marking to diagram 1028.3 or 1032, each with
the legend LOADING ONLY, or 1033 with no legend.
The bay to diagram 1028.3 has the largest maximum
width (3600 mm) and is therefore the most
appropriate for goods vehicle loading. Where the
loading bay is used mainly by cars or small vans and
has been provided to allow customers to collect large
purchases from nearby shops, the smallest width
of bay to diagram 1028.3 (2700 mm) might be
adequate. Diagrams 1032 and 1033 are also more
appropriate for smaller vehicles.
The bay you were in is not to diagram 1033 and therefore the bay you were in requires the Loading Only legend to be marked on the road.
A clear point for appeal.