This is just the Blackadder writers having a joke. You may have noticed that the video opens talking about Richard IV who, of course, never existed. It's not really a metaphor for anything, just a joke along the lines of Monty Python's mud farmers from Monty Python and the Holy Grail.

For reference, in 1486, the lowest form of currency in England was the farthing, worth a quarter of a penny. There was a new issue of farthings in 1486, as coinage had to be produced featuring the new Henry VII, but it didn't replace anything, other than the previous issue of farthings bearing the name of Richard III. 15th Century farthings are very rare, although whether this is due to political upheaval, successive re-issues, simple lack of production, or the coin's small size hampering its chance of archaeological survival is unclear. For reference, over four million farthings were produced during the reign of Edward I, but very few survive today.

Until the introduction of copper coinage in the early 17th century, the farthing was a very small coin, as it contained only a quarter of a penny's worth of silver. The first farthings appear to have been introduced in the early 13th century, although the term was in use far longer. Indeed, Early Medieval English penny coinage from the 10th century onwards frequently featured cross designs on the reverse to enable the coin to be easily split into halfpennies and farthings for small-value transactions.