Lang
My interpretation (for what it is worth) is that in referring to 'case by case', 'case' relates to the context and/or service not an individual.
For example, an organisation that runs leisure centres, rather than making a blanket decision about trans people being able to chose which changing facilities they change in, they should consider the facilities in each context. For example, they may chose to apply an exemption where there are open changing areas in a leisure centre but not in a centre where there are only floor to ceiling cubicles (I know that this is still unsatisfactory to many- I'm using it to highlight my point).
I can't see how it can refer to individuals because it would be illegal to ask to see GRCs and very undignified and intrusive to ask trans people about their bodies.
I think where it is appropriate to provide single sex provision/services they could simply make it clear that changing facilities are sex segregated by natal sex and to provide some mixed sex. Like in many aspects of life, people are required to follow the law (only drink in pubs once a certain age, drive having passed a test etc).
I know that some TRAs ridicule notions of 'policing' single sex provision. It would be the same as age restricted provision or driving licences, if suspicion is aroused, checks could be run and consequences faced if someone is not following the rules.