That's fine and if it come to it I am sure schools will do their best to put things in place, but for many schools it won't be easy. Time and staffing will be the big constraints, as well as space. Many schools, especially those in old Victorian buildings, already struggle. Some of our classrooms are linked and open - it is to allow free flow for the children, so no doors and only half walls between them. I dont think this is that uncommon especially at reception and infants.
Lunch time is often already staggered, even then you generally have a large number in one space at a time. Playtimes - its a case of timings and staffing.
We would probably need staggered start and end times too - most schools only have two or three entrances to their schools and often have 100+ pupils and parents coming in and out of entrances and playgrounds in close contact twice a day. Staggered starts are going to mean parents, as well as schools, change their work patterns.
Its all workable but needs a bit of planning to sort first, and will probably rely on staff being willing to give up own their unpaid breaks and lunchtimes , etc - so working for free. For a lot of our classroom based staff (the teachers are slightly different, though they'd probably be need to be looking at getting rid of PPA time to aid the staffing, which is prescribed non contact time) this will mean up to 45/60 minutes per day they may be asked to work for free.
At my school we are already doing a lot of pre planning for if and when there may be staff absences, closures, etc, I spent all day yesterday working on the contingency plans for getting work out to pupils, and allowing staff to work together from home, for the instances of 'well' self isolation. Unfortunate most schools, especially, primary aren't really set up for working for home and distant learning - so a lot of our planning and prep is looking at how to sort that, as we already know school closures are possible. Ive been working with advice from an ex colleague who is having to do just that, with no real warning, for her school in China - which has been closed all month.
As the Government has already stated this could be a plan, parents also need to be trying to plan in advance what they could do to help themselves - not int he case necessarily of self isolation, but more in the case of schools closing for 2-4 weeks. So, maybe forming 'teams' with parents sharing the care with one another, allowing the to at least go into work for some of the time. Again, not ideal but just starting to think of ways round it IF it is at all possible.