V&A cafe has 3 rooms, all of which were designed by top 19th century designers, including one by William Morris. They are well worth a look for architectural reasons, and the food is pretty good too.
If the weather’s nice, you can also have a drink outside in the quad.
If you’re visiting the V&A I can’t see why you’d go anywhere else unless you were looking for a real treat (as noted, Bibendum).