Complex sentence: He also met and became friends with the son of the British ambassador for Greece Spyridon Trikoupis, and future Prime Minister of Greece, Charilaos Trikoupis, who was just starting his political career as an attaché, then "chargé d'affaires", of the Greek legation.
Split into simpler sentences: He also met and became friends with Charilaos Trikoupis-the son of the Greek ambassador to Britain Spyridon Trikoupis, himself destined to become Prime Minister of Greece. At the time they met, Charilaos Trikoupis was just starting his diplomatic and political career as an attaché, then "chargé d'affaires", of the Greek legation.