My guess is that by tomorow this will be backtracked as the amount of parents in an uproar will likely be alot.
I think this is likely too. Like many things, if you're the only person affected, you end up suffering; but if it's loads of people, the combined outcry almost always forces them into retreat. This is how it worked with the rocketing fuel prices and likely will again next winter, if the government don't put measures in place (assuming the fuel prices don't tumble back down, which would be the best solution, of course).
The whole 'school isn't childcare' excuse is ridiculous: parents obviously, quite reasonably, plan their employment around when their children will be in school in term time; and for universal credit claims, parents of school-age children are expected to be in (or actively seeking) work when their kids are in school, as there are teachers in loco parentis during school hours, which is exactly what childcare is, to all intents and purposes.
The head sounds like an absolute fool, highly irresponsible and unfit for the job if they are incapable of planning the school's timings (including appropriate notice periods for changes, preferably following parental consultations and full impact assessments) in any kind of acceptable way.
It's especially tone deaf during this period of upcoming strikes: I'm not saying that the teachers shouldn't be striking, but how would they react if you gave them one week's (or more) notice that your child WILL be in school for the standard term dates over the next term and, if they find that they need to close the school at any point, it's their problem to find a solution and not yours? What's sauce for the goose etc.....
If they are insisting on going ahead with this, the very minimum that they need to do is guarantee free breakfast club provision during term time until the end of the school year. If they can't do this at such short notice (especially as it is their choice to make the change and thus not short notice to them), how on earth do they expect you to accommodate it? I agree with PP: make it crystal clear (ideally together with other parents in the same boat) that it's their decision to make the change and obviously not realistic (if even legal) for you to be expected to be able to accommodate their preferred changes at such short notice, so what plans have they made in order for their changes to work?
Clearly, as somebody in the highly responsible position of a headteacher, they WILL have plans in place for such a radical change - so what are those plans, please? And, just to remove any shadow of doubt, "I dunno" or "You will have to find a way to accommodate my wishes" are not in any conceivable way acceptable plans, as the LEA will surely 'agree' (whether they like it or not).
Seriously, this is much more akin to the (lack of) thought-process of a teenager in their first job after leaving school, who blows all their wages on 'cool stuff' on payday and then suddenly realises that they still have outstanding bills to meet and food to buy for the whole month; not a mature adult in the trusted position of a school headteacher.