I believe they were also shortened at the "country" end
Correct, P24 most notably AFAIR, the trackside 'face' of p21 being the one that remains mostly as it was originally built (from memory again, so will stand corrected!). They were shortened in order to allow for a more domestic style track layout to be installed (crossovers etc) between the inbound home signal gantry (in to P21-24) and the platform ends. Said gantry is itself positioned London side of Carlisle Lane Junction, ie: on the short section of self contained (international) station approach roads, hence the platforms had to be shortened at the Country end.
The old Eurostar track layout that once existed in the International throat, London side of Carlisle Lane, was completely un-compatible with the requirements of the planned domestic services that were to use those platforms.