I would question you being skint on 40k especially if you are in the nw.
Thats a decent wage for a family even down south. Too many people go on about that wouldn't go far down south but I've spent my life down south and it is down to areas and yes there are people in London and other areas of the southeast on less than 30k. They survive and some even thrive.
It's a mindset. But 40k is decent, you need to shop around for everything and cut back on needless items but you are able to save.
I think only people on the lowest income levels cannot save and those with high expenses like rent and childcare that they can't easily change. Because moving is expensive so that old trope of just move cannot always work.
But everyone else can save, they just choose not to. £2 a week or something but it's possible. Start small.
If you are serious op post a statement of affairs to moneysavingexpert.com, debt free wannabe. Loads of great advice there and read the threads on how to live a frugal meaningful life while still enjoying your life.
To that poor man upyhread MitchellR , I think you should have cut off the account a long time ago, 7000 dollars in 3 months and your wife still has access. It is not financially abusive to say no and to cut off the funds when someones spending is so out of control.
I have suffered financial abuse and you would not be doing so by saying no. You need to say no as you may find yourself unable to pay rent or homeless and you have kids. Also you need to have a serious conversation no matter what her reaction is and set out a budget.
Otherwise, you may as well get divorced as she is showing no respect for you and is being very selfish but you also need to act.