Decimalisation, or Decimal Day as it is most known, occurred in 1971 after the pre-decimal system was deemed too complicated with pounds, shillings and pence. Initially, the plan was to make the new decimal currency into cents and dollars (known more in the USA), but this was reconsidered, and pounds and pence were the chosen coinage.
Queen Elizabeth II: Pre Decimal vs Decimal Coins
There were 12 pennies to the shilling and 20 shillings to the pound. There were also Half Crowns, Three Pennies, Sixpences and Farthings. Prior to the decimalisation era, many had problems dealing with the complicated system, with Sir John Bowring, a Member of Parliament at the time, being one of the first calling to change to a currency based on units of ten.