HESH does not need to penetrate - it works on the Newtons Cradle effect, like those executive toys with 5 balls on a cradle you used to see in gift shops. It "pings" a big scab of steel off the inside of the armour plate that it hits, any penetration is a bonus and only applies to thinner plate.
Its really only useful on
homogenous steel armour, or bricks and concrete (which it reduces to individual grains that being why it was used in the original "wallbuster" RCL gun experments of WW2) and is highly susceptible to any gaps of any kind or the armour having layers of non-similar materials (such as rubber or sand or angled plates etc) within it. Spall liners like you see commonly on modern armour will also mitigate the scabbing effect as well.
So in game, any HEAT armour counts as triple the value to HESH after the steel is subtracted
Armour v HESH = HEAT + ((HEAT-Steel)*3)
HESH is better than HEAT when being used as an HE round, but is still reduced compared to plain HE as HESh is a thin-walled case that produces blast but little fragmentation effect.
(I can give that detail out since HESH is
our code, not SSI's
)
Why the UK was so wedded to it for so long is a mystery. HESH/HEP really only excels in an engineer demolition gun type application now that tanks have had layered armour since the 60s and the T64s appeared.