You are more fortunate than many, as at least you have detached yourself emotionally from him. "All" that needs to be sorted are the practicalities.
In the first instance I would gather together all the paperwork you can get your hands on about the finances. Most especially the savings he has squirrelled away. The starting-position in a divorce is 50/50, so that's half of everything: house, savings, pensions, assets, the lot. With two children to support on your own it's possible that you could get a greater percentage, especially if you've been out of the work-force and sacrificed your career prospects to raise your children while his prospects have been enhanced by you taking on all of that load alone.
How are things arranged financially at the moment? Do you have a joint bank account into which both of your salaries are credited and from which all bills are paid?
Get onto Rightmove and see what suitable properties might cost you if you choose to leave with the children.
Have a look at the the "turn2us" website to see what benefits might be available to you should you need to claim them. In theory, he will have to contribute 20% of his income in child-support and that money is not taken into account when calculating any benefits you might be entitled to. However, given that he's in control of your finances, unfortunately you will have to anticipate that he's unlikely to be co-operative in the first instance but once you are formally separated he's unlikely to be able to get away with totally withholding it from you.