Picking up on the Unova representing all of America part, wasnt it said somewhere the name meant
UNited states OV America
To me Sinnoh really felt like the UK, even the thing about time and space starting in Sinnoh and Mt coronet being a line down the middle just reminded me of the Greenwich meridean line.
So I dont think theyd choose UK for Europe, possibly France or Germany. Italy would be good as would Spain, and yes the Ancient Greece Idea would be good too, lots of Islands for Nintendo to use.