Everything you've written is believed by you because you have never been divorced before, I take it.
Neither have I. It's amazing what you believe about divorce until you do it for yourself.
Not unlike all the things you believed about having babies before you did!
What you need to do is speak to a solicitor. Yes it will cost you a few hundred pounds - however it will a) dispel a lot of the myths you believe right now and b) stop you panicking.
Yes, it will also upset your spouse. But given they are already upset - there's nothing to lose there.
You cannot take your husband to the cleaners.
Your husband cannot take custody of the children if you are the SAHP just because he earns the money.
With a £1m home, there are 'lifestyle' factors to be considered. Yes, you will have an income drop, yes you will be expected to work (part time until they leave school), and possibly yes you may have to move homes - but this is by no means a certainty.
Your husband cannot just abandon the marriage. Yes, he can upsticks and fuck off, but the law doesn't give a shit where his physical body is, it will ensure his financial responsibilities to his children are present and correct.
Once 'needs' are established - e.g. school fees, bills, mortgage payments, the rest is 'lifestyle'. He doesn't get to keep 100% if that, but clearly you can't claim 50% of it either. That's where it all gets very interesting.
I currently get 15% of my husband's income (following a interim hearing) - but that's more enough to keep the wolf from the door -but I can't afford a holiday, but it's still a lot more than minimum wage. This won't last forever, but until we finalise the divorce.
Ergo, until you divorce you are both obliged to maintain the norms of your marriage financially speaking. So if either of you start suddenly acquiring incurable shopping habits, or disposing of assets (or selling them for a £1 with a give back promise) you will suffer for it when it comes to the Final Order.
Do not start cutting the shirt sleeves. That too will get you in a lot of hot water.
So if your husband is threatening to cut you off now - or leave you with a lot less than you need (need not want!), then you too will apply for an interim hearing pending final order.
Divorce law is extremely complicated. You can afford a solicitor - and in fact looking at how naive your post is (and I mean this kindly - I could have written it myself a few months ago!), a solicitor is going to be the best decision you can make.
It does not make you a bitch. It forces you both to parent.
Parenting is not just kissing the kids goodnight. I'm sure YOU know that. HE, on the other hand, can fuck off.
I hope this helps.
I recommend you read 'Family Law' by Gordon and Slater. It explains things to the layman.
Every divorce is unique so what I get in my divorce may well be nothing like what you get in yours!
And when he's acting like a madman - I suggest you pop into my thread and write a Dear STBXH rant. It's rather therapeutic.
Take care, and I'm sorry you're in this situation.