Is this a financial mistake? Help please

I would not rely on your ex saying he will pay that much. He might be willing to do it now, but what if he changes his tune?

I don't think the money is enough for school fees, unfortunately. I personally would move areas, and invest the 80k into a new house.

You can’t afford to send her.

You could save or invest the money and reconsider for secondary, but you absolutely cannot afford to send her for primary, if that’s what you are getting at.

Fees increase at a much higher rate than wages.

Maintenance is not guaranteed - he could lose his job, move abroad, take a lower paid job, have multiple other children. I’d be inclined to rely on it for a much shorter time (such as seniors) but you would be relying on neither of your financial situations changing for 15 years, and that’s not realistic.

£80k isn’t going to fund an entire private education, and if you’ve spent all your money at primary level, where it’s much easier for your involvement in their education to fill in any gaps, you’ll be stuck for secondary where it isn’t and find the step into the state sector more difficult than if you’d been state all the way through.

Honestly, unless you need to pay down the mortgage, if you really want to benefit your DC, invest a good bit of that £80k and give them a tidy deposit for their own property: that’ll push them ahead far more markedly in their adult life than private primary school.

The mortgage. Your little one is only 2, the first few years of school they don't need to be private, I don't think you really get value for money there. Having a low mortgage gives you freedom.

My son is in a good school in an ok-ish area, the childrens behaviour (and the parents) isn't ideal sometimes. But we do extra work on the evenings and weekends and he is ahead of where he needs to be.

I think this area will be just fine for primary, and I am focused on going to a grammar or looking at private scholarships potentially for secondary school.

Your kid is only 2. Invest the money and see if it grows enough for private secondary education

No, you won’t be able to upkeep those costs.

I wouldn’t go private on your income. The fees will go up every year and there’s extras to pay too. The £80k won’t last long if you did that.

I personally wouldn't spend on a private school, I'm simply saying that because you asked what I would do.

However, it sounds important to you, so if it would make you and DD happy, that is a good enough use of the money. Assuming you're one and done of course.