'Case by case' needs challenging.
It's not in the EA itself, the first place it crops up is EHRC's statutory code. While this code carries a lot of legal weight, it's not the law. It can be challenged if it can be shown to misinterpret the law.
It's not clear where 'case by case' came from in EHRC's code. Their post consultation report says they talked to 'a diverse group of expert and specialist stakeholders' (so not service users).
Women's orgs they consulted:
An-Nisa (assume this is a Muslim women's group?)
Chwarae Teg (Wales)
Fawcett Society
Maternity Action
Scottish Women’s Aid
Women's Resource Centre
LGBT orgs they consulted:
a:gender
Accord Coalition (Ireland)
Gires
Press for Change
Stonewall
Stonewall Housing
(worth noting - this is before Ruth Hunt at Stonewall so they were probably mostly still advocating for gay men rather than trans)
From 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.
From 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.
So it looks like 'case by case' in EHRC's staturory code has come about because of pressure from trans lobbying groups, most likely Press For Change and GIRES.
More than 'proportionate means', more than 'legitimate aim' (both of which appear in the EA itself), 'case by case' scuppers our chances of using the single sex exceptions. It turns any legal fight we might bring into a personal dispute against this particular tw and why we have sought to exclude this particular tw from a female only space.
'Case by case' means service providers have no right to provide female only space, even where this is a proportionate means to achieving a legitimate aim. You can have a female-only policy but you still have to consider whether it should apply to this particular tw - every single time.