A law on its own does nothing, you need an enforcement mechanism
I think in many (even most) single sex spaces proper use of EA exemptions should be enough to protect women. Women can/do sue their employers or service providers under the act if they don't apply the exemption correctly and potentially the organisations owning or running those spaces are liable.
For many privately owned spaces women will vote with their feet/wallets and just not use the space if males are there, providing some pressure to keep males out if the companies want to stay profitable.
Public toilets are different as they are by nature open to the public. So maybe a new "toilet law" would be needed as there is no way the toilet owners control access. I really don't understand how a toilet law would work or be paid for, which is partly why I think toilets are a bad place to start this debate.
In the case of public toilets (I.e. the service station) what's the mechanism once the toilet law is in place? Do women report to the police to investigate, is it security guards/CCTV, what? Do offenders get an FPN or a criminal record if they breach the law? Do you think councils should be spending money detecting men in women's toilets (at a time when council funding means public toilets are increasingly shutting down, which disproportionately affects women and limits participation in public life).
Do you think the police should divert effort from other crimes into detecting and prosecuting men using women's toilets? The police already can't resource catching men looking at child abuse images. What do they drop to investigate and build a criminal case against the man in the ladies?
Do you think parliament should spend time debating then legislating for this? What things would you take off the legislative timetable instead?