Hi
If you don't want to go to each website individually, the only other method I can think of would be to find other reports already made which cover gender inequality in corporations and collect the information from there. But if you want something that keeps being updates and is accurate, the bets would be to write a script (code) that would check the relevant page of each company every few months. Yes, it would be a lot of work finding all the pages in the beginning, but afterwards at least the updates would be automatic. I know a guy who writes great scripts if you need.
Good luck
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I am not aware of any other way of getting this information apart from looking at websites and making a freedom of information request (UK)