It was HESH most probably, British Army never used HEAT for L11A5 gun, even if there was designed one, I think it was exported, and is used by Chieftain users in middle east.
However there ae rumors that this Iraqi tanks was destroyed not by HESH but by APDS/APFSDS.
It is hard to tell what is truth in this case, also no one mentions how many rounds crew fired before they hit enemy tank.
Firing unguided munitions at such distance is problematic even for tanks with modern FCS, and Challenger 1 had hardly modern and good FCS back then.
Well, probably soon I will be eaten by rifled guns lovers for that blasphemy.:roll