Taken from BS7671:
A cable concealed in a wall or partition at a depth of 50mm from a surface of the wall or partition shall:
(iv) be mechanically protected against damage sufficient to prevent penetration by nails, screws and the like or
(v) be installed in a zone within 150mm from the top of the wall or partition or within 150 mm of an angle formed by two adjoining walls or partitions. Where the cable is connected to a point, accessory or switchgear on any surface of the wall or partition, the cable may be installed in a zone either horizontally or vertically, to the point, accessory or switchgear. Where the location of the accessory, point or switchgear can be determined from the reverse side, a zone formed on one side of a wall of 100mm thickness or less or partition of 100mm thickness or less extends to the reverse side, or
Summed up this means: If the vertical cable run has no capping, it is okay as it is inside a zone, however as the cable runs at the bottom of the wall horizontally it is outside of the zones and needs to be mechanically protected.