"What does that legislation mean then!"
It means that pay should not be determined by sex, which applies equally to both sexes.
What is does not mean is "Zero Gender Pay Gap", which might be what you are thinking it means?
There is a right in law to "Equal Pay" and that right needs to be defended. It also needs to be exercised in order for it to be of benefit.
For example, if pay grades are not published and employees do not know how much pay other employees are receiving then inequalities can go unnoticed. Job Evaluation Schemes can be a good thing, in making pay schemes transparent, but they can also be rigged and used by employers to obscure inequalities and defend unfair differences in pay.
If someone believes that their employer is discriminating against them on the basis of their sex, in that their pay is less than a comparator employee of the opposite sex, they have the right to apply to take a case to an Employment Tribunal.
If you belong to a Union then the Union Lawyers will examine the case and will advise whether or not the Union should support the claim. Unions will normally only support claims that have a reasonable chance of success.
www.tuc.org.uk/join-union
Other options are to go it alone, pay for legal support or use a "McKenzie Friend".
www.legalchoices.org.uk/types-of-lawyers/other-lawyers/mckenzie-friends
If the Employment Tribunal agrees that the claimant is suffering a detriment on the basis of their sex then it can order the employer to equalise the pay, with back-dating to the time of the claim.
It is rarely an easy case to make though because the claimant and the comparator are unlikely to have identical contracts of employment (Job Descriptions, Terms & Conditions).
This is a handy summary, with much more detail on the rest of the site:
"On this page you will find a summary of equal pay and its relationship to the gender pay gap."
www.equalpayportal.co.uk/equal-pay/