Axis and Indigo in the One Aldwych Hotel are good choices for pre- or post-theatre dining, and are quite reasonably priced.
Rules is nearby too in Maiden Lane.... as is the Savoy Grill....
Bank Aldwych, corner of the Aldwych and Kingsway, is perenially busy and appears quite popular with decent pricing. I have not gone yet but friends say the food is okay but does have a nice buzz to it, though the volume gets quite loud around Happy hour.