Caroline Lucas is not the first Green parliamentarian. As readers have noted, Lord Beaumont of Whitley sat as a Green. Having been active in the Liberal Party, he left the party and joined the Green Party in 1999 and sat as a Green in the Lords until his death in 2008. As Teithiwr notes, Cynog Dafis was elected to the Commons in 1992 on the back of the support of a coalition of Plaid Cymru and the Greens, but sat as part of Plaid Cymru until he left the House in 2000.
It was John Bright who coined the phrase ‘Mother of Parliaments’ and he was referring to England, not Westminster.