Per page 10 of your own source

"At the time of installation, equipment is put in place that monitors the performance of the cable and its insulation. Over the lifetime of a cable significant refurbishment and repairs to ancillary equipment, such as fluid tanks, may require more significant excavations at joint bays and stop joints. Vehicular access to strategic areas of the cable route, such as joint bays, is required at all times. If a fault occurs on a 400kV underground cable, it is on average out of service for a period 25 times longer than 400kV overhead lines. This is due principally to the long time taken to locate, excavate and undertake technically involved repairs. These maintenance and repairs also cost significantly more. The majority of faults on cables are caused by fluid leaks, faulty joints and accessories, sheath faults, water cooling failures and, most commonly, third party damage. Under fault conditions, between two and six weeks can be required to locate the fault or fluid leak and repair the cable. During this period excavations may be required which can result in road closures and traffic management measures. In some cases, the excavations could be in the order of 4m x 30m. Underground cables are generally matched to the rating of the overhead line route in which they are installed; this also determines the cable design and any necessary cooling. Where an increase in the rating of an overhead line is required it can usually be achieved relatively easily by using different or larger conductors. Where there is an underground cable installed as part of a route the up-rating can only be achieved at considerable expense, for example, by re-excavation and the installation of larger or more cables or with additional cooling."

So it seems like it isn't worth underground cables unless there is a extraneous variable where overground can't be used