And on the childcare costs, it isn't a case of him not paying. Our salaries all go into a joint account, so we would be paying collectively., or if I were to stop working we would also take that financial hit collectively.
It still makes more financial sense in the long run for you to keep working, especially if you're not married.
Let's say you and your partner were both on the same level workwise now:
Situation A: You stay in work
You both put into the family pot, you pay out for childcare - it's tight for a couple of years but you're both building your pension pots, you're both gaining experience and length of service in your positions, you pick up legal protections, maybe entitlement to extra days of annual leave, incremental pay-rises, opportunities to upskill/train and advance even if only through seniority.
Say, 4 years down the line, you're both likely to be on a better wage and with better prospects, a solid working history and a fatter pension pots. You decide you can no longer stand each other and split up. You're in a decent position financially and workwise.
Situation B: you become a SAHP:
You give up work, you save childcare costs, but for however long you're out of the workforce, you're not putting into your pension pot, your skills go out of date etc. 4 years down the line, if you can find a job, it's likely to be the same level or worse than when you left the workforce. And because your partner will be effectively 4 years ahead of you in his work life, he's able to pull in a better wage, so it still "makes sense" for your job always take the backseat so you end up going part-time or being unreliable because you always have to drop work for the kids. So your pension pot stays a lot thinner than his, and your earning power and prospects are miles behind his.
You decide you can no longer stand each other and split up. You're up shit-creek financially and workwise.
Alternative:
If you marry, you have additional protections and rights which will help if you become a SAHP and then the relationship founders.
Another alternative:
If you both take the responsibility and both continue working, but work flexibly or both work part-time and share childcare between you, then the strain and opportunities are equal for both of you.