Your pension is unlikely to be equal to his - put it all on the table
Your potential income is unlikely to be anywhere near his - get evidence of his that you are entitled to - so if paid into a joint account or declared for mortgage etc
The starting point for financial order is 50/50 and then they look at needs, future income etc
If its beneficial to stay in school and finish this year/next year and that was the plan spusal maintenance in the short term could be a possibility
Remember any investments like shares or prem bonds or sep savings accounts
He WILL hide money so go for your share of what can be seen
As others have advised - get your relative to write a letter setting out the terms of the loan and get the money back to them asap. Make sure it was a loan for a specific purpose and its clear
Benefits folks look back over a few years for deprivation of assets these days
YOu have made sacrifices that scuper your working - rural living, giving up work etc etc but he has in terms of you being in school etc.
Don't be doing everything around him - make sure you get yourself out in the evening and he does bedtime and childcare so you can go talk things over with friends, visit parents, sort your issues out
I would say stay in the house but if its too rural thoughts need to be given to where might be best for you - have a look for housing. If there isn't much equity this is going to be difficult - but check out shared ownership style housing, which while you have to be careful about which management company you go with can be a godsend if you need 3 or 4 bedrooms and single person - you pay rent on the part you don't own but it is significantly less than market rents at the moment
Practically start thinking about what you will need over the next 6 months clothes wise for the kids - school uniform??, even things like toys - sort them and keep back good condition toys and clothes for sale later.
Start deciding what you are wanting to take with you to provide for the children. I might also think about a small storage unit to keep things like paperwork safe and away from the home if things get nasty - kis passports and BC and yours. Any legal paperwork and also any valuables etc
Think about the car situation - you will need a family car and don't need to be scratting around with 2 bit cars. BUT bear in mind this is viewed as an asset for benefits purposes and the value will be counted towards your savings limit.
All in all whilst UC can be helpful if you have a reasonable amount of assets its useless and you need to keep your options open - if you can get any assets into a accommodation for you and the kids then it doesn't count on your savings total. I would suggest being in a town, near enough to schools to walk, near to shops etc that you don't need a car for now.
Is school on line or OU or ??
Investigate possibility of child minder type care as you will find he won't want to have the kids except at weekends as it "interferes" with work
ALl of this is worst case but that is what you must prepare for, if it turns out better you can be pleased