Not sure how true this is but someone told me if you go off with PGP and they can't find you lighter jobs then it can't trigger early mat leave, might be worth looking into. Or have you any annual leave to take?
They can trigger your maternity for anything pregnancy related. If you can get signed off with something that is not related to pregnancy then they can’t send you on maternity early
I was signed off for a few months due to PGP but had to move annual leave from the end of the maternity leave to 4 weeks before it was due to start to save triggering the maternity leave early. Other option is you could say you aren't coping with the hours and they can reduce them?