Susan Foreman, being a time traveller, had visited Britain's future prior to her arrival in 1963. Confused, she could not remember how many shillings were in a pound when she and the First Doctor settled down in London in that time. She thought that they had already decimalised the currency, to the amusement of the other students at Coal Hill School. (TV: An Unearthly Child)
The currency was still used during the far future. When Hame was very young, her parents sold her to the Sisters of Plenitude for 10 shillings, so that they could afford taking themselves and Hame's siblings out of Restitution. (WC: The Secret of Novice Hame)
Behind the scenes Edit
Interestingly, with An Unearthly Child set and broadcast in 1963, Susan accurately predicts that the United Kingdom would switch to decimal currency, which in the real world was announced in 1966 and occurred in 1971.