I will move this out here now to avoid too much thread detritus (apologies, may also be one for a modwomble as well).
Efficiencies of scale is a bit of a red herring, there is a hard coded limit to how efficiently you can pay a teacher at a certain point on the pay scale.
The issue would then be where you start with the extra spending. You could pay teachers more, you could do that and increase the numbers of teachers so the workload is reduced (more PPA time, although you still want your good teachers actually teaching). Support staff pay is woeful and the more senior staff there (SBMs etc.) are getting towards breaking point so that would need to be addressed.
You could then think about reversing the reduction in provision of “unviable” subjects at GCSE and A Level which puts the breadth of curriculum back or keep smaller (often rural) schools open. You could also stop some of the reductions in the initial SEN interventions that make teaching possible as they are by necessity next on the chopping block for schools who need to cut somewhere.
Oh, and you could do school rebuding at a greater rate than having an average lifespan of 400 years.