RGS appeals may well be hard to win - but that depends on the strength of the school's case (and quite possibly on the relative strength of all the other appellants' cases too).
Just to be clear - if you go to appeal, you're not
appealing "against rejection based on distance
The admission criterion is irrelevant - just so long as it's been correctly applied.
You could live a very long way away, but win your appeal if you have good enough reasons for seeking a place.
my understanding of over subscription appeals is that you have to prove this is the only school for your ds
That sounds more like the test to be applied when the school has a social/medical criterion. This is usually applied very strictly by the admission authority.
For example, a child with a medical/social condition could well be refused admission (a) on the basis that this is not the only school that could cater for the child's needs, and (b) none of the other criteria are met.
However, if that child's case went to appeal, the outcome is then determined by a balancing exercise.
The Appeals Code clearly sets out the test to be applied where oversubscription is concerned:
"The panel must balance the prejudice to the school against the appellant’s case for the child to be admitted to the school. It must take into account the appellant’s reasons for expressing a preference for the school, including what that school can offer the child that the allocated or other schools cannot. If the panel considers that the appellant’s case outweighs the prejudice to the school it must uphold the appeal.