Has any detailed planning / analysis been conducted on which parts of the country and indeed individual councils are most likely to see private school pupils moving to the state sector, and whether the state sector in those particular areas could absorb this increase in numbers?
Talking about spare capacity in the state sector as a whole is a non-argument if the movement is likely to be concentrated in particular regions.
Something like 85-90% of Local Authorities have more spare spaces than they have private pupils, with the others being fairly close, and easily able to absorb much more than even the most dramatic of Daily Telegraph predictions.
The only real outlier is Surrey, but even there they could absorb over a third of their public school pupils. I'm pretty sure no-one is going to (non) argue that a complete collapse of the Surrey public school system is likely. Obviously within a borough, the demand may be spread out, but it's unlikely to be so distorted as to cause unsurmountable problems. Anyway, public school kids are used to travelling longer average distances to get to school, so maybe even that fear will turn out to be a red herring.