I'm the queen of practical.
Money, if you have joint accounts then freeze it, move half of your savings somewhere to your name only.
Get as much paperwork as you can, statements, mortgage, marriage certificate, birth certificates, all bills and just put them somewhere safe.
Search through his stuff. Especially his payslips, recent or his P60. Get copies of all his papers then put them back, copies of as much of his stuff as poss.
I expect there is someone else for his sudden behaviour. He is currently on Mr Guilty but it won't last. So get the proof of his earnings.
Don't argue with him. Disengage. I know you want to scream why why why at him. He won't tell you. He made choices and you weren't invited to discuss them. So now you do the same. Make your choices to move your life forward without discussing them with him. Use Mn and friends but do t bother talking to him. Ignore him, put all communication on email. Don't txt unless it's pick DC up at 5pm. Be dignified and kill him with kindness. Even though you will at moments hate him more than you thought hate possible.
The solicitor when you get there will want to know all the money stuff, equity in house, cars, credit cards, debts. Has he got secret credit cards, his own bank account? It goes both ways that his debt is your debt. As well as assets.
Divorce is a long process. To petition (be the one who requests) you pay a court fee of £400 ish. Lawyers fees are another £4-600+ so just to petition with a lawyer is over £1000. Generally. The other person is the respondent (replies to the request of divorce).
There is little point is accusing him of all sorts as then the lawyers earn more in discussing your wrong doings in letters. So be amicable with him over your grounds for divorce. Unreasonable behaviour that your communication has broken down and the marriage is irrecoverably damaged is plenty enough.
If you argue via lawyers then expect large lawyer bills. Get to mediation (cost around £300-1000 depending how quickly you can agree) and then both present your lawyers with your findings.
If you can't agree and he is stitching you up, then your lawyer can attempt to get him round by writing to his lawyer. Ultimately if it doesn't work you apply for a court hearing (must go to mediation first). Court costs a lot. Judge makes a decision and everyone sticks to it.
Within the paperwork of divorce you should have lawyer make a court order to detail the agreement of money and contact for the children. It's not necessary but safer for the children in the long run. So for example, he pays £X every month until oldest is 18. Then £X till youngest is 18.
If he deviates you get it fixed quicker, and if he wants to change it he has to apply to the court again.
So petition for a nisi (ends the marriage) and then send for an absolute (when everyone is happy with the finances and childcare).
Don't stay with a lawyer you don't like, go talk to a few, pay them if need be to meet them and take their advice.
If money is really tight then good luck. Try to find a lawyer who will give you free advice. Ultimately that's a sales proposition. My lawyer is quiet, factual direct, concise and particular, he suits me!
Sorry so long!