The pins on a BS 1363 plug are 4 mm (0.16") thick and 6.35 mm (0.25") wide.a factor of insulating plugtoptop (sic) pins must make some difference to heat loss due in part to the insulation itself and also the reduction of CSA of the pin conducting mass.
If the thickness of the half-insulation on a BS 1363 plug is 0.6 mm in total thickness (both sides), there would still be 3.4 mm thickness of metal
- at least 80 %
A 4 mm * 6.35 mm "pin" has a CSA of 28.6 mm²
Reduced to 80%, this is still 22.9 mm² - More than Eight Times greater than any 2.5 mm² conductor which may be "feeding" them
and
almost Six Times greater than a 4 mm² conductor!
Also, all "Conductors" are incased in "insulation".
Hence, if a 2.5 mm² copper conductor encased in insulation is "Good" - for 20 A,
a 22.9 mm² "copper alloy" conductor encased in insulation is "Better".