It's unusual to see SWA with an earth wire, are you sure it's not
concentric. Consentric carries the earth and the neutral in the
armouring,
with the strands that carry the neutral coated in a PVC sheath. Typicaly
concentric will have no brass glands on each end of the cable.

TN-C earthing is not permitted within a consumer's installation. It is
permitted on the main supply line to the property, provided the earth is
separated off into TN-C-S within the supplier's cut out.

Christian.