Interesting discussion here on the lowest buying power of any British coin in circulation:
https://www.bbc.com/news/business-48153442
They start their measure from Queen Anne's reign and the farthing, which is a little odd as the only farthing made in that reign I understood was only really a pattern?
Also, they make no mention of the third and quarter farthing coins, which I suppose is more reasonable as these coins were really intended for colonial use, although legal tender in the UK as I understand it.