To some extent even if you weren’t pregnant it would be financially better for both of you if you divorced.
His income which he spends with little thought is holding you back.
If he wasn’t living with you, you could claim benefits and CM off him.
You would have 1/2 the equity of the house and he would then have less expenses and more disposable income.
I have a relative who although they loved their dh and he loved her they found it financially unviable to remain married.
They were always close and for many years they lived separately as bf and gf, co parenting the children until they had enough income that they could manage with out the need for benefits
Sometimes you have to be practical.
From what has been posted i don’t think you want to divorce.
If so I think there are areas where you can cut back if you choose, there always is.
Can you explain about him working 3 days per week in London. Is this part of his ft job or does he only work 3 days per week in London?
Have you got out the statements and looked through them with a fine tooth comb.
When Dp told me we were skint after he had been looking after the finances for a few years I went through our expenses and everything from mortgage to phone contracts and every little expense like having a newspaper delivered.
In one day I saved £1200 per month.
Is there anything you can do to earn a few extra pounds. I have friends who make jewellery or trade on eBay.
One made so much money from her pin money business that her dh and then dB gave up their jobs and now work ft in the business.
Even matched betting would bring a few quid extra.
Ultimately though is your dh completely against having a 3rd regardless of your income or is he saying that the finances make it impractical and you can’t afford a 3rd.
With regards to the effects on your children.
I have about the same gap as between what would be your 2 youngest and they are firm friends even now they are both late teens.
Your eldest might act jealous or she might not.
Girly days and making sure she is included in the upbringing and treating her like the big sister is all you can do. The rest is up to her