Wow. I'm in the UK too. What struck me when I read this was that the issues you describe with your children, you're the one thinking about how to help them, not you and your wife, yet they're her children too.

You can't fix everything all at once. Yet trying to do things step by step is difficult, because it's all connected. So I would work on a support plan first. Financial and practical. Do as much preparation as you can. In terms of savings, accommodation, understanding where you sit legally, finding out about mental health support networks, looking for a good therapist...... all of that. It's like building a house from scratch. No one just goes ahead and does it without a lot of organisation first. You will always find plenty of reasons to struggle with this (GCSEs and so on) but there's never a good time. The situation sounds unbearable and the longer you leave it, the harder it will be. So start your research! Here are some websites

Information & support | Mind, the mental health charity - help for mental health problems
https://www.england.nhs.uk/mental-health/adults/iapt/
Divorce | Divorce Support Group | Getting over a break up | Professional Divorce Help.
https://gingerbread.org.uk/content/433/Community
https://fnf.org.uk/help-and-support-...ranch-meetings
Divorce Recovery Workshop ? National Helpline: 0333 335 0489 (local call)
https://www.wikivorce.com/divorce/

Good luck!