I have just done an experiment. I only had the heels of Warburton toasty small sliced bread. I grilled the crusty side, then turned it over and added baked beans, cheese, dried herbs, then back under the grill.
It was great! I think it was actually better using the heel rather than the non-heel!
All these years I actually thought the heels were just there to keep the rest of the bread slices soft, I thought everyone threw them away!