"What Took You So Long?" debuted at the top of the UK Singles Chart, with sales of 76,000 copies. This made Bunton the fourth member of the Spice Girls to top the chart as a solo artist, and also set a record for the group, as they became the act with the most individual members to have their own number one single in the country. It spent two weeks at number one, three weeks inside the top 10, nine weeks in the top 40, and 13 weeks on the chart in total, ending at the 41th position on the 2001 year-end chart. The song was certified silver by the British Phonographic Industry (BPI) in the UK in April 2001. As of January 2021, it has sold 295,000 copies in the country.
