I immediately got out my "Too May Songs By Tom Lehrer" song book,and couldn't find the song either.I spent some of my teen years growing up in London and remember well the thrupence ad farthings.
At that time,1959-1962, the exchange rate was $2.80 to the Pound so the 10s note was $1.40 and so on.
And for Wolfang's benefit he wasn't the only one using pennies. An American penny was exactly the same size andweight of the six-pence piece which was about a dime. They worked perfectly in the candy machines and I could get my dosages of Cadbury's Fruit and Nut,cheaply.