@Blobby10
It probably won't be any comfort to you but I've often found the exchange date is agreed after the completion date - its almost as though having the 'final' date gives all parties the proverbial kick up the backside to get stuff sorted!
I think I get what you're trying to say.
Exchange can only take place once all parties in the chain agree upon a completion date.
Once all legal matters are concluded, all contracts signed, deposits lodged and a completion date settled on, then exchange can take place.
Exchange doesn't happen on a date that has been pre-agreed as such, it happens on the earliest date that all of the above has been dealt with.