On the other hand :
"For this reason, encouraged by European initiatives, 34 of the 48 Countries participating in the Green Card system* have replaced the Green Card by a Multilateral Agreement recognising each other’s MTPL insurance on the basis of the “territory in which a vehicle is normally based”. Consequently, for motorists from those 34 countries, the Green Card is no longer a required document when crossing the borders."
Greece and the UK are among them.
Ah well, just another unresolved Brexit complication, I suppose. Sigh.