Who is Thales in London?

Samuel Johnson’s London prominently features the character of Thales, whose discontent with London is causing him to leave imminently. Thales and his friend, the poem’s narrator, are in London on the banks of the River Thames waiting for a boat to take Thales away. He is headed for Wales on the southwestern edge of Britain.
As they wait, the two converse about the current state of English society and politics. Thales declaims passionately about the dreadful conditions that he feels compelled to escape. He names a range of ills from petty vices through systemic corruption. As a young man, he sees no future fulfillment will be possible. The poet uses this character as the mouthpiece for his critique of English society, especially the Tory government of Robert Walpole.