It looks like you're using Internet Explorer 11 or older. This website works best with modern browsers such as the latest versions of Chrome, Firefox, Safari, and Edge. If you continue with this browser, you may see unexpected results.
European Commission webpage with some useful information. The Fawcett Society also carries out research on this topic for the UK,
as does the OECD for OECD countries (data from 2000-). See the Gender Pay Gap report from UK Parliament published as HC 584, session 2015/16. ONS has Gender pay gap tables from the Annual survey of hours and earnings.
International project, based on research by Piketty, Atkinson & others, providing data on various aspects of income for a growing number of countries worldwide, with statistics back to the 19th century in some cases.