There is no mention in the current admissions code about needing to prove that you have sold your old house. Nevertheless, one can understand why the LA would want to be satisfied that your move is genuine.
There were quite a number of newspaper reports last year about parents trying to play the system, particularly in Hertfordshire and East Sussex. The LAs were coming down very hard on them, and using all sorts of methods to investigate parents who were suspected of giving falsified evidence or moving temporarily to gain a school place.
I dealt with a case last year where a family had moved to Bucks from quite some distance away. Despite the fact that it was patently obvious that the child could not possibly travel to and from school each day from the old address, and there was nothing obviously suspicious about the case, Bucks CC still demanded very substantial amounts of evidence to prove that the original house was being sold and that the new home was a permanent move.