OP, manage your expectations on this one pl.
All the regs say is that as regards a 'box junction' this may only be placed and is defined as a)at a junction between two or more roads;
As far as I am aware there is no case law which clearly lays out what is and is not to be considered as where two roads meet. *
If you've sent your reps then the die is cast and you can only await their response.
If this goes further then you might want to look at the layout of other box junctions in the council area e.g. do they all follow the same pattern of being placed at the point where the radius ends as it meets the parallel side of the road being joined, as here. Or perhaps, as the Traffic Sign Manual (TSM) shows, the corner of the box should be placed at the mid-point of the radius. Another factor could be that the 'give way' road markings in the side road terminate prior to the yellow box, indicating that this is the correct extent of the 'junction'.
*- in simple terms, is the radius inclusive or exclusive to the side road or, as per TSM, they have an equal share.