But how is there no money left for food?
You said the pay increase after tax was £300 a month, and the rent and council tax increase was £330.
So - it's almost neutral.
I think it's especially a problem as your marriage is in trouble anyway.
But leaving that aside, I do think you're finding barriers that aren't there. Like the cost of formula - your baby is 8 months. In just 4 months you won't be using formula.
You haven't explained the long term prospects.
Would I trade a bigger house in Glos for slightly less money a month and no further progression? No.
Would I stick children in same room and re-work how the sleep (they'll adapt!) or keep baby in cot in my room for two years knowing that my husband's salary would increase and I'd have more work options myself? Yes I would.
Honestly though, if you're in counselling now I wouldn't move. Send him off on a weekly board basis, or renting a room. The £300 a month will go a long way to paying for that (about half). If you divorce you don't want to be stuck away from family support.