Would promotion involve a lot of extra hours for you? If not (or if it's manageable), would it be worth just taking the promotion and stuffing the whole damn lot in your pension for now?
I know this wouldn't improve your current situation, but my thinking is that it would then put you on the path towards the next promotion or next job move which might get you over the point where it starts to make a difference to your take home pay (at which point you stop stuffing money in your pension like a crazy lady, accept the tax man eating that chunk, but at least start seeing some more money each month).
I think salary sacrifice for an EV would also be a way to pull it down and would improve your family life if you are having to get rid of your old car. Assuming your employer has a scheme of course.
Thanks for the suggestions. Per this budget apparently now I could stuff enough from the next promotion up into pension without breaching the £100k of post-pension earnings and therefore losing tens of thousands in net pay and losing the house. We'd still get no more money as net pay from me taking the promotion which is what we desperately need and is not allowed, but as a stepping stone it might be worth it IF we could trust that the higher pension contribution allowances would be maintained while I was stuck in that situation. Then it might just about be doable - to get no more money now but put it into pension and just work work work for the next promotion when we might actually be allowed to keep some of the money in my monthy salary in a few years time followimg a second promotion.
But, if I took the promotion then Labour reverse the pension changes as they have pledged to, if in power in just over a year, those higher pension contributions would not be allowed, then huge cliff-edge effective tax rates would kick in for breaching the £100k threshold where we'd lose tens of thousands of net pay overnight. We'd be screwed and lose our home, even though on paper I'd have a higher salary than now, because I can't actually access any of that money as net pay because of this tax system. I just can't take that risk as the sole provider for two children, that politicians will just take our home away (for the crime of being promoted!). Although I suppose that'll happen anyway if Labour get in if they raise taxes further on anyone earning over £80k as they said previously. I genuinely have no more money to pay any more rises. There's nothing left. Labour don't seem remotely interested in equalising the tax system to give single parents the same allowances as couples, any more than the Tories are. Nobody is on our side.
We'd only be better off in terms of actual take home pay if I get two promotions, and I'd need another 4-5 years to get to the second one. I'd have to sacrifice yet more time with my children in what's left of their childhoods after they've been through a huge trauma already, on a hope and prayer that politicians won't reduce pension contribution limits again and by doing so make us homeless and bankrupt while I work for the second promotion. It's just too risky.
Benefits like EVs still count as taxable income so sadly that wouldn't help escape this cliff edge where if you hit £100k you are hit with an extra tax bill of tens of thousands of pounds, just for earning £1 more.
It is such a mess. I want to carry on with my career and do more for my children but until these tax issues are solved we are paralysed, treading water and desperately trying not to drown.
Thank you for your replies though, and to everyone who's been supportive. A lot of the time it's like speaking to a brick wall and nobody cares so it means a lot and I really hope more women will stand up politically to campaign to end the tax discrimination against single mothers, it does so much damage.