Can i appeal to my second choice school if i get allocated my first choice.
Yes, I think so.
What are our chances of a successful appeal?
Impossible to say. If it were the other way round, I'd be warning you that Bexley appeals are thought to be quite tough!
You could write to the admission authority for the school you're interested in to ask for the number of appeals to the school in each of the last 5 years, and the percentage succeeding. You're entitled to this information under Freedom of Information. That will give you an idea of what has happened in the past, even though it doesn't guarantee what will happen in future.