My understanding is:
under the Equality Act single sex anything is generally not allowed because that is discrimination against the protected characteristic of sex. So a men-only business network or a vacancy for bar work that requested women only would probably not be ok.
it is however allowed for something to be single sex if it is propotionate to achieving its aim, like a woman-only refuge. Possibly a woman-only business network if it could be shown that women in that industry were a minority and struggled to build effective networks.
What the SC has clarified is that if a single sex exemption is being invoked, the sex on which inclusion/exclusion is based must be biological sex not GRC-certified sex. If you don't do that, you don't have an exemption and then I assume you fall back into not being allowed to discriminate based on sex at all.
Now, what I think is interesting also is that while gender reassignment is protected, gender identity I believe is not. So in theory it's totally fine to have a "woman-gender-only" event just like you could have a Goths-only night - neither is a protected characteristic.
BUT - would a woman-gender-only event hit the problem of indirect sexism? Because genderists have made such a fuss about "cis people" and "cis normative" being the majority and trans the minority, could a man who does not identify as a woman claim he is being indirectly discriminated against on the basis of sex, which is proected, because most people who identify as women are female?
Wouldn't it be hilarious (but also actually genuinely progessive) if this means gender identity ends up being totally disconnected from sex, "cis" and "trans" go away altogether and it's just personality preferences like it always should have been?