As I've said @RitzSpy, there's nothing wrong with using temporary buildings. Except that they are temporary. Our town hall, for example, is licenced for weddings. Now, when they open, are they supposed to tell their wedding parties that they can't use the building because the school is? Or our local museum is supposed to turn people away because the school has it.
Using a field or park is fine (forgetting safeguarding for a moment), but as soon as it rains what happens to the kids? Told to stay at home? Our secondary has children travelling by bus for 45 minutes. What happens if they arrive at their field and the rain starts? How do they get home (if home is where they need to go).
All these temporary things are fine (to some extent or other) but there is a need for permanent solutions. We could be like this for an academic year. Firefighting has got to stop and proper plans put into place. Our school easily has room for a row of 'nissan' style huts like the RAF/Army used during the war. But they don't have cash to buy them.
That sort of 'innovation' would also help with the scenario of having teachers next door to TAs rather than one working in one building/one in the other. And if each set of porta-cabins had a loo block, your handwashing problem is sorted out. But cash needs to be found.
If we can furlough people to the cost of billions, we can find some to rent porta-cabins or other semi permanent buildings for our schools. Asking for rooms/fields/parks to be used is fine for the next couple of months, but October? Mid February? In a field? Really?