I would very much recommend the Children's Educational Advisory Service. We had some issues with special needs provision for our daughter when moving from the Scottish system back to England on posting and what appeared to be a huge issue with the LEA was solved in one phone call from CEAS.
I know people who had issues coming from overseas, particularly one involving someone returning from an embassy posting in a strange in fairly similar circumstances that you describe and CEAS sorted them out and even managed to get the child a place at an even better school than they ever thought possible.