From what I recall the government case argued (as one of their points) that the SEISS was designed and rolled out at short notice, and needed to be implemented quickly. Therefore they couldn't make adjustments for Mat leave as it would have made the process/calculation too complicated.
So they were "reasonable" to do a one size fits all approach given the constraints.
What I don't get is how that argument can still apply. Fine, day 1 you have to roll out something urgently and naturally with a predominantly male team and the male default no-one thinks about women who might be self-employed and taken maternity. But it's been months, and they have had plenty of feedback and evidence that this was a serious oversight.
Maternity and Pregnancy are stand alone protected characteristics alongside sex, how can they justify having not made any amendments in all this time when they have made substantial revisions to other aspects such as furlough?
Through maternity allowance they do have the data on who has taken maternity leave and women could provide other supplementary evidence such as MatB1 forms etc. All women are asking is that instead of a straight average of 3(?) years they are allowed the exclude those years of lesser earnings due to maternity. Even backdated now this would help a lot of women.