I'm not sure how you've done your calculations, but I have no idea how you could staff a 5-form entry primary school on that budget, even with cheaper inexperienced teachers. Not after you've factored in NI and pension costs which are obviously impacted by the pay increases as well.
As for our financial management, we've had external "experts" come in and look at it (voluntarily through ESFA and an independent commissioned by the governing board). They have concluded that the budgets are managed tightly and that the only way realistic way to reduce costs is to reduce staff costs. So we either need to get rid of our experienced and expensive teachers or cut back on support staff. We don't want to do either but sometimes tough decisions have to be made.
We also benchmark against other local schools. We know that the proportion of revenue budget spent on leadership team salaries is lower than average, and that our average teacher cost is higher than average. We spend a higher proportion of our overall budget on teacher costs and we have a higher staff retention rate. I don't know what we can do about that unless we proactively find ways to manage staff out once they get too expensive, or start treating staff like shit so that they leave of their own accord. Given that we believe that the quality of education on offer is heavily dependent on the quality and wellbeing of our staff, neither of these are seen as reasonable ways forward.
Our primary focus at the moment is on developing income generating activities to help plug the gaps, but those are long term measures that won't resolve the immediate pressures on our funding.
Perhaps the situation at your school is different and cuts to support staff don't need to be made. But all schools are different and I can only speak for the one that I know well.