You do not have to apply any extra 'safety margins' to allow for the characteristics of ordinary fuses or circuit breakers; they have already been taken into account in the tabulated rating of a cable. For example, a cable rated at 13A will be adequately protected by a 13A fuse, or put another way, whatever a 13A fuse will withstand before rupturing, so will the cable.
The general principle to be observed is:
IB<=In<=Iz
where
IB = design load current
In = rating of protective device
Iz = tabulated rating of cable, multiplied by any applicable correction factors for temperature etc.
Actually it is a little more complex than this under some conditions and with non-standard types of protective device, but that rule holds good for normal cartridge fuses and domestic circuit breakers.
Quote:
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I bought a 50m drum of 2.5mm TRS cable... I found that the 13A plugs and extension sockets available at the time (1976) would just accommodate the cable.
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You might have had success installing that cable in some brands of accessories, but to the best of my knowledge BS1363 never required them to accommodate it. Judging from your picture, the sheath radial thickness is less than that of a modern H07-series cable that is usually specified for heavy duty outdoor applications, so it might have been more obliging.