As an HR professional, I would say that this is very risky territory for your Company.
At very least, I would expect someone from senior management to have explained the situation to you and to have confirmed that there is no loss of status etc, issued a new job description or to have confirmed that your job has remained unchanged.
Factors to consider:
How large is your Company? Do they have a professional HR Department?
What sort of communication did you agree to when you went off on maternity leave? How much contact have they had with you? When you went on maternity leave, what was put in writing about your return? What sort of correspondence has there been since and what did they put in writing when they confirmed your return. The larger the Company, then the higher the expectation and onus on them to act professionally and have sophisticated policies and procedures in place.
Status is an important part of terms and conditions being 'no less favourable'. By that, if your new boss is now attending meetings that you previously attended as a Manager then that is a demotion. Your job cannot be materially of a lower status. It would all hinge on how your current job description and whether that has materially changed due to the changed reporting structure. An example might be if you managed a team, held budgets, were responsible for achieving certain business objectives, were involved in monthly reporting and as such, attended senior management meetings as a member of the senior management team. If your boss now has all these responsibilities then that 'could' constitute a demotion and constructive dismissal. If you were able to argue that you have been treated less favourably because of your sex and the taking of maternity leave, then there is no limit on the compensation that you could be entitled to. I must say though, that this is a highly complex area of law (proving constructive dismissal and sex discrimination) and I would urge you, if you are thinking of going down this route to get a good Solicitor who specialises in this area.
However, I would add that this sort of situation is quite common, in my experience, with many of my friends being 'made redundant' during maternity leave.
You seem to be taking it well- best of luck!