Tables for current-carrying capacity and voltage drops of old Imperial-sized British cables, as per IEE Wiring Regs. 14th Edition, 1966.

Table 3:  PVC sheathed twin & multi-core, copper

Table 9:  Butyl & silicone twin & multi-core, copper

For comparison, here is a table from the previous Regs.  This is taken from the ASEE Guide to the 13th Edition, 1955, as amended to December 1963:

Table 13:  Twin & multi-core rubber/polythene/PVC insulated cables