[Lucien Nunes]

AFAIK these switches used the same components as the switches in their consumer units, of which the common types were rated at 60A but there might have been a 45A version as per @Tractorfan (or was it 40A?) There were other, similar-looking versions such as the immersion one and 3-pole metalclad 20A type with a wider toggle, some of which we still have in use here in the workshop

6mm² or near imperial sizes were not unusual for meter tails especially in the days when 40A cutout fuses were common. In fact two single-core 6mm² are more or less rated for 60A and many cutouts are still 60A today. However the convention is now to install 25mm² allowing for future 100A service upgrades without having to change the tails.

Depending on whether the total load served by the tails exceeds their contiuous CCC, the service cutout fuse upstream may only ever need to provide fault protection, not overload protection. For example, an additional CU with total outgoing In (OCPD nominal rating) of 50A in theory only needs a supply cable rated for 50A, provided it meets the adiabatic requirement to avoid thermal damage at the prevailing fault current. This can often be achieved with what appears to be a remarkably small cable, compared to one rated for the full upstream OCPD In.

Taking a modest PSCC of 1kA and 70° PVC insulation with a k constant of 115, and a 60A BS 1361 fuse that achieves <0.1s disconnection at 1kA, the minimum meter tail CSA that would be required in order to be protected against short-circuit by the cutout fuse would be:

CSA = sqrt(I²t)/k
= sqrt(1E+6 * 0.1)/115
= 2.8mm² minimum.

For that reason, old small tails feeding individual switchfuses, as were commonly installed to serve a 30A ring final circuit when socket-outlets were modernised to 13A, have often remained safely in service. 7/.044 was common, or 7/.064 for a larger consumer unit.