Government report just out analysing the gap and the reasons for it. Lengthy read but accessible and really interesting
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachmentdata/file/706030/GenderrpaygappintheeUKevidenceefromthee_UKHLS.pdf
"We find that if women had similar amounts of full-time work history as men then the size of the gender pay gap would decrease by 43.6%."
"We find that if men and women took similar amounts of unpaid family leave the size of the gender pay gap would decrease by 13%"
"The next biggest factor behind the GPG concerns all observed and unobserved characteristics systematically associated with being female, accounting for 35% of all drivers of the GPG. This is equivalent to women earning £0.57 less per hour because of observed and unobserved factors associated with them being women. While we are unable to definitively identify why ‘unobserved’ criteria continue to be such a large driver of the gender pay gap, it is likely to be a function of an unknown combination of discrimination and gendered behaviour (constrained or otherwise)."