If you are moving to England the options available to you depend on the school and possibly also the local authority. They must consider your request to hold back your son a year properly. They cannot simply apply a blanket policy. However, most LAs and most schools won't allow you to hold him back a full year unless there is evidence of significantly delayed development.
Your son does not have to be in school until September 2018 so he doesn't have to start this year. However, if the authorities do not agree to him being held back a year he would go straight into Y1, missing Reception completely.
There are other options available to you through the state system. You can delay his start until later in the school year. He would still go into Reception but wouldn't start until January or April 2018 (or, indeed, any other date before the summer holidays but most parents opt for a start of term). If you want to do that you need to apply for a place as soon as possible then, when you've got one, tell the school that you are deferring entry. It is your choice, not theirs. They cannot refuse.
Whenever your son starts school you have the option of sending him part time until September 2018. Again, this is your choice. Some schools try to imply it is up to them but it isn't. If you want to send your son part time they have to accommodate you.
Would you even have a school place for September
Yes she would. She may have to push but, once she is in the UK, the LA is required to find a school place for him.
whether you should have applied this year and deferred
She could not have applied this year as she is not yet in the UK. She still has the option of deferring until later in the year.
You might not even be able to get a deferred reception place
She will be offered a place, although she may have to push the LA hard to make it happen. Once she is offered the place she has the absolute right to defer entry or send her son part time.