@Bitesize89
Whether or not you'd like to raise grievance against her depends on what you'd like to achieve. In my experience, these things are stressful and rarely go in favour of those raising grievances as the HR departments are largely there to protect the company and its management/leadership from its employees, not the other way around. This may sound harsh but I found this to be true.
In consultances, when there is a project that someone would like you to work on, you do usually have a choice to accept it or wait for another but I can also say that if there is no alternative to projects offered to you, bench time (esp. prolonged bench time) will always look worse than any project regardless of what it would give you in terms of personal development, career aspirations, satisfaction etc.
On the subject of workload, consultancies tend to be pretty brutal. I don't know when you returned from your mat leave but if it was less than a year ago, then any prospects of a promotion are pretty unlikely.
Even if you returned a while ago, if you struggle with your workload, this can be easily interpreted as struggling to perform at level and therefore any prospects of a promotion are also remote because most people considered for promotions in consulting firms work long hours, which are well beyond their contracted hours, and usually outperform their peers at the same level.
I don't want to sound cruel but this is my experience of working for these firms.
Now for the positives: management in consulting firms tends to change often, so if you would like to stay in the company rather than leave, you can either wait it out and see if your managers would change or apply for another role.
If you want to leave because you can't take it anymore, then I don't know what the value of pursuing any kind of formal process against your manager would be.
My suggestion would be to consider carefully whether consulting firms are the right places for you now that you have a young child or children or maybe if not consulting firms per se, then client facing consulting as a job (I assume that is what you are referring to).
Wishing you all the best!