Can't see anything wrong with it myself, although others may disagree. It appears to comply with diagram 1044 in Ch. 5 of the Traffic Signs Manual (p77) and with Para 12.10 (p.78) which says "Half -boxes, in which only half of the junction is marked (diagram 1044) are appropriate at "T" junctions and other junctions where the traffic blocks back from one direction only. Half-boxes may be used only in the position shown in the position shown in the lower drawing in figure 12-1, i.e on the minor road side of the main carriageway.". I believe you may be thinking of a full box junction which is Diagram 1043.