Legal aid is now only available where there's proven dv or child abduction.
Not all solicitors are equally good so get recommendations from friends who've been through a divorce either their lawyer or their ex's if they were good.
Some may offer a free half hour but it's by no means definite and to be honest it's not necessarily the better ones that do, but the ones struggling to get clients.
Gather all legal and financial paperwork (including evidence of all his income) and keep safe.
Does he still have access to the house? If so you need to consider if you can legally prevent access and if not any sensitive items need to be secured elsewhere (not just paperwork but valuables and sentimental items also)
Do you have joint bank accounts? If so is your money still going into them?
I'd recommend opening a new account in a completely different banking group. Get any income you get paid into your account ASAP.
Cancel any dds/standing orders for items that were just for his use while at home ASAP eg sports tv packages.
Move bills that need to be paid to run the home to your account too so you know they'll be paid - while he's feeling guilty and remorseful he may agree to pay these but that often quickly wears off and the next thing you know your phones cut off!
Look on the various benefit eligibility calculators tomorrow to get an idea of what you might be able to claim
https://www.turn2us.org.uk
https://www.entitledto.co.uk
But bear in mind that the results are only a guide.
You could also post on moneysavingexpert.com
Monday, if you think you would be eligible contact the relevant agencies to start your claims as they're only backdated to date of first claim NOT when he moved out.
This also applies to child maintenance, again while guilty/remorseful he may promise all sorts but when that wears off you could face problems and the sooner you contacted them the better, even contacting them doesn't mean you can't have a private arrangement if that works for you but they're slow so if you are going to need them best to get the ball rolling.
If money is going to be right look at your current budget and see where you can make savings.
Sorry about this one but...you also really should get a full sti screening. Not pleasant but necessary, even if you've been using condoms.
DO NOT tell him anything you don't absolutely have to, he is not your friend or ally any more.
Contact, courts are moving more towards 50/50 but it depends on many factors, children's age, whether you both work and what hours, what accommodation is available for the children, if they're in school etc
It IS a lot to take in but you WILL manage you'll get through this.
Sorry for the shock you've had and good luck 