Can we still use our head office location for back-to-back trades (where we don’t store the material)?

If you are a seller initiating back-to-back trade located trade with stock physically located at one of your sites, you should use only this site, not head office. If you are back-to-back trader who is selling stock held at the seller’s site, your material will be linked to this site. It would appear automatically (No need to choose a location for you as a back-to-back trader).