I've been a single parent for six years now. My kids were in school when I left but I've still had to find after school childcare and holiday childcare up until recently. I also had to pay for childcare on my own even when we were together as my ex, like yours, refused to see it as a shared cost or responsibility.
Universal Credit will contribute to childcare but it is in arrears, so you have to pay it and then provide proof of payment. It's fiddly but if you can get the timing right and your childcare costs are regular, it's doable.
Depending on how much maintenance your ex is paying, you may be able to get more by going through the CMS, though they're pretty useless at enforcement through anything other than salary deduction.
Some employers are more understanding than others; personally, I think it's worth trying to find a family-friendly one, whatever the sector. I worked for the public sector for a local council and they were very flexible when I needed time off for sick children etc.
Another option is to try to reduce your hours slightly, say 4 days a week rather than 5 to cut the cost.
There are options, none of them are easy but I promise it's doable. I now have one at secondary and one about to go to secondary, so childcare is less of an issue now. It's not forever!