Paragraph 28 provides an exemption from liability for discrimination as to gender reassignment, when exclusion is a PMoAaLA. Excluding Buck Angel from a support group for rape survivors would be a proportionate means of achieving the legitimate aim of providing a psychologically comfortable environment for group members. It's always been possible and it's not controversial.
I believe there may be a wider point arising from the preceding two paragraphs, which establish an exemption from liability for sex discrimination under Sections 13 and 29 for service providers who wish to provide service users with a single-sex space as a PMoAaLA (safety, fairness, modesty etc).
The Section 13 definition of discrimination includes perceptive discrimination (case law). Therefore the exemption from liability under paragraphs 26 and 27 applies. A woman who looks like a man can legally be excluded (without additional justification) from a space provided for the exclusive use of women. And this has always been true.
This might sound shocking, but makes sense to me. These spaces have always operated by means of self-policing, superficial appearance, and mutual consent. Women are good at accurately sexing other humans. And in the days when male interlopers were rare and socially disapproved of, they might give odd-looking women the benefit of the doubt (no doubt thereby sometimes letting in those 'you can't tell' male transsexuals we keep hearing about). But they would be entitled to turn those odd-looking women away.
Who might they have been? An example would be a woman with a DSD who has a female birth registration but has at least partly gone through male puberty. (Yes, she's karyotypically and gonadally male, but under British law she's biologically female, at least since W v W.)
(Such a woman was a claimant in the GLP JR application, and had been understood by colleagues for years as 'intersex'. Do not get me started, on the unethical way that GLP used her in their grift, instead of helping her to sue her idiot employer, which had banned her from the ladies' after FWS.)
So, those odd-looking women (I can remember a couple from my childhood and I'm pretty sure they weren't trans)? A bit undignified and annoying for them, but they could always carry a driving licence or passport, just in case.
But not any more, because the sex marker on a driving licence or passport can now be changed.
OK what about photo ID plus a birth certificate?
That now only works with a certificate issued before the age of eighteen, so too bad if you've lost it!
(A certificate issued after the age of eighteen could be from the GRR and therefore is not proof of sex.)
So the gender ideologues have made it genuinely difficult, for a tiny group of people who actually might need to prove their sex, to do so. Way to go! They made things worse, not FWS.
And why didn't it occur to anyone that being able to prove your birth sex is still necessary, even on the most 'trans-friendly' reading of the law?