"after several phone discussions with the School Admissions Office, we were assured our child’s secondary application will be accepted by using our UK address"
@ReturnfromtheStars As a pp said, if you have this in writing it will help you at an appeal.
"However, on 1st November my husband received a phone call stating we have to put our overseas address on the form, and the application is now deemed incorrect and won’t be accepted as an application on time, but instead will be treated as a late application."
The forms usually make it clear that you have to use your current residential address, not an address you plan to move to. If you have a screenshot of the form, showing the wording, it may help.
"I have found a guidance document for Surrey (sadly we don't live in Surrey)"
This won't help. Every area handles these situations differently. You need to use the procedures from your home area.
"Admission authorities cannot, for example:
require the applicant has a permanent home address either in the UK or overseas before processing an application"
This won't help. The local authority have processed your application and found that you have not used your current residential address. Fom their perspective, this is potentially fraudulent, so they have rejected your application and asked you to correct it. They haven't refused to process it.
"If a local authority refuses to co-ordinate an application for a school place from parents currently living in another country but moving or returning to England, an admission authority for the school would have significant difficulty in being able to lawfully apply its admission arrangements."
Again, they have not refused it. They have asked you to correct it. You need to resubmit it using your current address in your home country. They will then process it. When you move, you can update the address.
"Surely, other people move before secondary too? Even if not countries, just counties. If it's a planned move I would assume they apply for school in the new place."
Yes, but they apply using their current address, then update it when they move. My local authority would only accept the new address after contracts are signed.
In your case, if you are moving back to a home you own, is it currently empty or let out? Are you paying council tax?
If I were you, I would phone the local authority for a chat with the admissions team, and ask their advice.