Stonewall succesfully argued in court that it wasn't their fault that people following their advice might find themselves on the wrong side of discrimination law.
I do not think it is a coincidence that companies and organisations have been walking away from Stonewall affiliations and the diversity champions scheme ever since.
Whatever Stonewall may think, they were seen as "best practice" for diversity, as grown-ups who gave grown up advice. Companies and organisations believed they were getting expert help navigating a contenious and tricky area.
That value proposition is now dead in the water. I am not sure SW realised just how much "winning" with that defense shot themself in the foot.