Pound sterling

The Pound Sterling was a form of currency used in Great Britain through the 20th Century. Its value is 20 shillings to a pound, and 12 pennies to a shilling. Susan Foreman, being a time traveller, had visited Britain's future before, and, simply being confused, could not remember how many shillings were in a pound when she and the Doctor settled down in 1963 London.

It would only be a few years afterward that Great Britain would switch to the decimalized Pound. Susan probably didn't remember which year it was that this occurred, resulting in her ridicule when she thought that they had already decimalized the currency while attending high school.