Morgana arched a brow at Arthur and looked at him with an expression that was nearing 'you've got to be kidding me'. First he cut her off before she could answer Gwen and then he was telling her to go. The nerve of him. Who did he think he was. She never let him tell her what to do in Camelot, order or suggestion or otherwise, muchless here.
However, Morgana was keen enough to think upon the situation. And she realized that both Merlin and Lancelot might not appreciate her telling someone. Even if that someone else was Gwen. Then again, they mightn't appreciate it because that someone else was Gwen. It was out of respect for Merlin that she gave her friend a small smile-that very much neared apologetic-and said, "You'll have to ask Merlin that." Rather than ignoring Arthur just to spite him.
Give Arthur another look, this one much more unreadable, she simply said, "Don't take too long." Before walking out, closing the door behind her with a very audible click.
Both for her irritation with Arthur and as warning to the boys in the kitchen that she was arriving.