11kV/400V TX with no LV protection

It is not relevant because this protection is not covered. BS7671 basically covers the LV side of installations, generally supplied by a DNO. Your private transformer puts you in the position of a DNO, and therefore you should at least examine their practice. A transformer is simply a device to change voltage, and the power on each side is directly related. As I have pointed out secondary currents are completely reflected in the primary, and this is the best and easiest place to provide protection against both overload and short circuits. The whole supply chain is yours, up until the HV metering. The BS7671 definitions of protection are therefore simply reflected to the HV supply, and there is no reason at all to not use this feature. The huge fault power available direct from the Grid is your main consideration, not the secondary cables, which you will find are very difficult to design with any normal protection anyway. Assuming the whole TX supplies one DB, the connecting cables are the least of your problems, the PSCC probably being the biggest. You will find it is probably 150-300 kA, and the weakest link is those connecting cables. HV fusing or CBs provides much the cheapest protection and anyway is required for your Grid connection.

It is not relevant because this protection is not covered. BS7671 basically covers the LV side of installations, generally supplied by a DNO. Your private transformer puts you in the position of a DNO, and therefore you should at least examine their practice. A transformer is simply a device to change voltage, and the power on each side is directly related. As I have pointed out secondary currents are completely reflected in the primary, and this is the best and easiest place to provide protection against both overload and short circuits. The whole supply chain is yours, up until the HV metering. The BS7671 definitions of protection are therefore simply reflected to the HV supply, and there is no reason at all to not use this feature. The huge fault power available direct from the Grid is your main consideration, not the secondary cables, which you will find are very difficult to design with any normal protection anyway. Assuming the whole TX supplies one DB, the connecting cables are the least of your problems, the PSCC probably being the biggest. You will find it is probably 150-300 kA, and the weakest link is those connecting cables. HV fusing or CBs provides much the cheapest protection and anyway is required for your Grid connection.