This is what I would advise (I used to be a union rep and have supported pregnant ladies with issues like this before)
Make careful objective (emotion free and factual) notes of every single incident with dates times and the names of any other witnesses if you can, as well as copies of any emails, texts or other evidence you have. Get copies of your employers disciplinary, grievance and equality policies if they have them, and read them. You can raise a grievance by yourself, by following the process in the policy, but if you're a union member you can ask them for help too. You'll likely need to write to your employer setting out your complaint and making it clear that you wish to raise a grievance. They should write back acknowledging it and set out next steps for investigation and resolution.
Decide what it is that you actually want to achieve by taking action, so for example, do you want the manager disciplined? Retrained? Do you want a sideways move so you can go back to work etc? An agreement that mat leave starts on a certain date and not before? Use the grievance process to negotiate this as a resolution to your complaint, being prepared to compromise if necessary, and bearing in mind they don't have to tell you the outcome of any disciplinary action against the manager, but should tell you if your complaint is upheld.
If internal grievance processes don't get you a sensible resolution, then check your house insurance, many policies include legal cover for employment issues, and if they do, or if you can afford it and you think it's worth the fuss, consider seeing an employment law solicitor to get some further advice. As you probably know discrimination on the basis of pregnancy does come under protected characteristics so it's possible you will have a case against your employer, but you wouldn't be able to take this forward without exhausting internal processes first.
As others have said Acas and maternity action are also excellent sources of advice too, and I'd fully recommend speaking to them. Bear in mind that any action you take will be fairly stressful, and will take time, and can have long reaching consequences. some of the people I've supported in the past wanted to just get their heads down and get on with sick leave/mat leave, and that's perfectly understandable. Others were determined that they would make their complaint heard and felt that it was worth the stress and hassle. Only you can decide what's right for you.