The Matrimonial Causes Act 1923
I am not sure whether this is generally considered a major leap forward in the advancement of women in England, but the writer Clemence Dane certainly believed it was:
[T]his slip of paper is one of the most important scraps of paper in the history of women, in the history, at any rate, of English women.... [it] defined, so long as England and the English idea shall endure, the status of women: it concedes for the first time her absolute right as a human being to the same law and the same justice that man enjoys. (Clemence Dane, The Woman's Side 1926, pp. 106-7)
I.e. it legally recognised a Single Moral Standard between the sexes, rather than having a much higher bar of marital misconduct in the male for authorising the dissolution of marriage. While it did not go anything like as far in establishing the wider grounds for divorce as some bodies had advocated to the Royal Commission on Marriage and Divorce prior to the Great War, nonetheless it was a significant step in making divorce more available.
It is not entirely easy to ascertain how far women took advantage of this new measure. In spite of the difficulties under the previous law a significant number of divorces had been sought and achieved by women. Under the new law (and possibly even before), there was a social convention that, if a couple wished to divorce, and were not on completely hostile terms, they would come to an arrangment whereby, whatever the rights and wrongs of the situation, the husband would let the wife divorce him by providing evidence of adultery. This would obviously somewhat exaggerate the statistics.
Even if he was the guilty party, the man might spare the reputation of the woman he might, after all, intend to marry once his first union was dissolved, by employing the services of a hired co-respondent, who would provide 'evidence' by going with him to a hotel, where the bill would reveal that he had taken a double room. and the pair would be discovered by the maid who brought the morning tea. The actress and nightclub entrepreneur Elsa Lanchester described in her autobiography, Elsa Lanchester Herself how as a penurious young woman she would undertake this masquerade - no actual sex was involved, they would usually play cards to occupy the time.
An instance of this 'judicial farce' was recently revealed when the papers of the solicitor who handled Wallis Simpson's 1936 divorce from her husband were opened in 2019. The hotel to which Ernest Simpson took his anonymous companion - 'the woman named' - was very proud of its reputation for discretion about its guests and therefore reluctant to disclose any information to the inquiry agent.
It was not until 1937 that AP Herbert's Matrimonial Causes Act finally extended the grounds beyond adultery, and even then the action remained adversarial and based in concepts of 'guilt' and 'innocence'.