It may not be enforceable on the basis of the workplace regs, but service providers (such as shops etc) are bound by the Equality Act and, since mixed-sex changing rooms are more of a detriment to women than to men, it is entirely possible that, if relevant legal cases were brought, allowing men into women's changing areas would (depending on the particular circs of the cases) be ruled as a violation of the Equality Act.
The real difficulty, though, is that once we lost the culture in which men routinely respected the boundaries of single-sex changing - and in which such routine respect was fully expected by society as a whole - it became unrealistic to expect retail workers to inform and remind men that they shouldn't be in women's changing rooms.
Retail workers have an increasingly shit time as it is, with customers becoming more aggressive and police less and less likely to respond to shoplifting and other criminality in stores. I can understand shops seeking to avoid situations in which employees have to take action to keep men out of women's changing places.
The real change will have to come from society as a whole recommitting to what was, until five minutes ago, an entrenched respect for social rules relating to women's spaces.