There's no need to wait, your unhappy now so start making plans. Doesn't mean you need to leave tomorrow, but start getting things quietly in place.
You may qualify for maternity allowance if you worked for 13 weeks in the 66 weeks before due date. The eligibility is here
www.gov.uk/maternity-allowance/what-youll-get
You will also be entitled to child benefit, make sure this is paid to you not partner. If they are a high earner, your partner may have to pay it back through tax. If doing this will put you in danger wait until you have left, but you can plan it in future budget.
Once you leave you will likely also qualify for universal credit, and with a new baby will be put in the no requirement to work category. Use an online calculator like 'turn to us' to get an idea of what you might get.
Tell your midwife and/or health visitor for older children, that you are experiencing domestic abuse. If it's documented then it can be used to put you in a higher priority band for council housing (as you can't safely stay at current home, so are effectively homeless).
Get yourself on the waiting list for council housing either way. Although it will be very dependent on area how quicklythey can help you.
Start squirreling money away, eg. If you get shopping, ask for £10/20 cash back and stash it. Then it's there for emergencies, and in time could cover rent deposit, if the council doesn't get you somewhere.
If you have someone you trust who can store stuff for you, get important documents and some essential out of the house. If they have the space you could also look out for anything offed on pass it on sites, that will be needed when you leave, and ask them to keep until your ready.
That's all the practical steps I can think of for now, but once you get through to women's aid, they will be able to guide you.
Best of luck to you, wether you leave next week or next year.