I've interpreted this reg differently myself. Look at (iii):
Regulation 522-06-06
A cable concealed in a wall or partition at a depth of less than
50 mm from the surfaces of the wall or partition shall:
(i) incorporate an earthed metallic covering which complies with
the requirements of these Regulations for a protective
conductor of the circuit concerned, the cable complying with
BS 5467, BS 6346, BS 6724, BS 7846, BS EN 60702-1 or
BS 8436, or
(ii) be of insulated concentric construction complying with
BS 4553-1, BS 4553-2, or BS 4553-3, or
(iii) be enclosed in earthed conduit, trunking or ducting satisfying
the requirements of these Regulations for a protective
conductor, or be mechanically protected sufficient to prevent
penetration of the cable by nails, screws and the like, or
(iv) be installed in a zone within 150 mm 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 100 mm thickness or less or partition of
100 mm thickness or less extends to the reverse side.
I interpret that as you either give the cable mechanical protection which doesn't require earthing or you cover it in earthed metal which doesn't give it mechanical protection. I haven't interpreted that as you need mechanical protection which is also earthed.
For the cable from the cooker control unit to the connection box I have had the cable diagonally but I have installed metal capping and drilled a hole for a 2BA screw which I clamped an earth wire to, then took that into the box.