I received a letter from the prime minister of the UK personally.
The person everyone comes to a mutual agreement about to be the manager of the country is the Prime Minister.
This is a jukugo word that uses the on'yomi readings of the kanji. You haven't learned this reading for 相 yet, so here's a mnemonic to help you:
The first Prime Minister of Japan was actually the shogun (しょう). That was before everything got complicated.