I think you will find that the regulations under BS 7671 indicate that the cable needs to be buried at least 18" or to be protected but not both.
Regulation 542-03-01.
All buried cables must be marked by cable covers or a suitable marker tape, and buried conduits or ducts must also be suitably identified. A warning tape above the duct or conduit run would be suitable.
Cables, conduits and ducts must be buried deep enough to avoid being damaged by any disturbance of the ground reasonably likely to occur. This could include gardening/horticultural works, excavating for any local buried service pipes etc. Generally, it is better to locate cables etc. clear of such possible works.
A depth of burial of less than 500 mm is usually inadvisable as shallow laid cables may be inadvertently damaged by general gardening etc. Cables that cannot be buried at a reasonable depth should be specifically protected, e.g. by ducts encased in concrete, or installed along an alternative route.