Copied and pasted from my link above:
"Establishing ‘home’ address
It is common for admission arrangements to give some degree of priority based on where an applicant lives. In these cases, admission authorities will need an address in order to apply their admission arrangements and rank applicants for their oversubscription criteria.
Admission authorities could ask prospective movers or returners where they will be living (see paragraph 2.5 of the School Admissions Code). This might include whether parents:
own or rent a property in the area to which they intend to return or move
are UK crown servants or are in the UK military and are returning or moving to the area
have provided other compelling evidence that they are returning or moving to the area
School admission authorities and local authorities can decide what evidence they require from parents to show that they intend returning or moving to the area, but this might include:
a mortgage or rental agreement for a property in the area
deeds for a property in the area
a letter from an employer showing a transfer date to the area
registration with a local GP
Admission authorities must consider all in-year applications and should not refuse an application simply because a parent or child currently lives in another country.
If a parent is unable to provide evidence of a return to the area (before the new school year for applications in the normal admissions round or for late applications, or by the start of the next term for in-year applications), admission authorities could apply a catchment area policy or distance tie-break, if they have such admission criteria, using the parents’ place of residence at the point the application is made. If this is in another country, it would give the child a lower priority for admission to most schools."