In the past, when I have ordered a list of items from Screwfix that exceeded the =A345 free-post trigger point, if some of the items were Nil Stock they would be sent later, post-free (for both Trade Counter and Web orders).
Apparently there has been a change of policy, and now you will have to order those Nil Stock items again. This will mean another visit the Trade Counter, or pay the postage costs if under =A345 and orderering over the Web.
CRB