Make sure your form is sent back in time for the due date or before if possible, (who or where did you read it needs to be returned 10 days before the due date?)
If you are waiting for information you can add a note in the extra section on page 19 explaining more information will be send and list what you are waiting for, and either that you will send it on to ATOS or take it to WCA appointment.
You can do a special or next day delivery make sure you stick the atos prepaid envelope to the front of your envelope and the post office will subtract the cost from your posting.
I have a form to be back by end of week which will be posted today.
Try not to worry and focus on getting all the information on your form and it sent off in time.