If you read the NHS homepage about EHIC card as the name is now it states
"The NHS is a residence-based healthcare system. Therefore, once you have moved permanently away from the UK you are no longer entitled to medical treatment under normal NHS rules."
This must mean that if you only will stay 6-12 month it should be valid. But perhaps mean that once you apply for recidence then you are theated as left UK. But I am sure someone else on here can tell you
"The NHS is a residence-based healthcare system. Therefore, once you have moved permanently away from the UK you are no longer entitled to medical treatment under normal NHS rules."
This must mean that if you only will stay 6-12 month it should be valid. But perhaps mean that once you apply for recidence then you are theated as left UK. But I am sure someone else on here can tell you