Paragraph 11.16 "with the publication of the Housing White Paper in February 2017 the definition of what constitutes affordable homes could be amended" This is clearly out of date and needs updating. Was the paper published last year? Was the definition amended?
There is no point in wasting time and effort worrying about a situation that has not arisen yet and may not arise. Since there are so many possible variables in the circumstances any such policy would have to be extremely comprehensive. Wait until a planning application is made and then assess it on its merits. If there is no formal policy in place then this would have to be debated by the Development Committee. You could meet the NPPF requirement by putting a reference to rural exception sites on the council's website.