It's rarely used because EHRC added the 'case by case' requirement in the statutory code after pressure from trans orgs.
From the Code (p198):
A service provider can have a policy on provision of the service to transsexual users but should apply this policy on a case-by-case basis in order to determine whether the exclusion of a transsexual person is proportionate in the individual circumstances.
From the Codes of Practice post-consultation report:
page 11:
vii. Various transsexual stakeholder groups responded to the formal consultation and also participated in the parallel consultation events taking place on the non-statutory guidance.
Feedback from the consultation events was incorporated into the employment and services codes where appropriate, particularly on issues of confidentiality, use of single sex services and the legal definition of transgender.
page 13:
e. Services, public functions and associations
• A number of concerns were raised about the exceptions, in particular the exceptions for charities, single sex services and separate services.
These sections have been revised as a result.
Adding 'case-by-case' (which appears nowhere in the Act itself) makes the exception virtually unuseable because it makes it personal. The service provider now has to show that it was a proportionate means of achieving a legitimate aim to exclude this particular TW. So we get into Tara Hudson territory and we can all see where that's led us to.
Most providers of the most desperately needed women's services are not wealthy, they are voluntary orgs run on a combination of donations, small grants and local authority contracts (which they have to compete for every few years). They know if they're taken to court they might end up bankrupt. They know that if they try to use the exception they will likely not win the LA contract (because the LA wants it done as cheaply as possible and don't want to have to fund alternative services for trans, or even for men).
And because those kind of services are the most 'extreme' - DV, rape crisis etc., it sets the bar for less 'extreme' contexts like changing rooms - if Women's Aid don't use the exception for refuges it makes it much harder for changing rooms to use it.
As well as the GRA consultation, there is currently an open consultation on Enforcement of the Equality Act which asks a lot of questions about the role of the EHRC.
It's not as straightforward as the GRA consultation but the response format is a lot less structured, so if anyone has any relevant experiences or thoughts, send them a letter.