As DG said, for the exam it doesn't matter where you live right now.
If you're going to be living in the inner catchment by 31st Oct, how close will you be? The maximum distance for non-exam places is given here:viewtopic.php?f=18&t=10227
In March, the boys Grammar offered distance places up to 332m and the girls to 727m. The girls distance has jumped up from the 250m of the past two years. It wouldn't be wise to rely on a distance place so far out again.
So unless you are going to be very close to the school you're aiming for, your child will still have to sit the exam.