Prezzo is another chain that would meet your needs.

We liked "Oriel" at Sloane Square -- just a couple doors down from the tube stop. It's a bistro that's open just about all the time but has a nice simple menu. Good value, lots of fun. On the "other side" of the river (if you've done the London Walks' Pub Walk you will end in the area, or if you've visited the re-created Globe Theater) try Tas, a Turkish restaurant with gently priced "menus" and a nice decor. We went to the one at 72 Borough High Street, but they have at least one other London location. www.tasrestaurant.com

Have fun!

There have been previous posts about Porters, and yes, I've eaten there. To be honest, if it weren't for their selling all those theatre and dinner packages and bringing in lots of tour groups, that mediocre restaurant would have been out of business long ago. It is like mediocre pub food at restaurant prices.
You would be much better getting half price tickets to a show and going to at least a half-decent restaurant. A lot of good suggestions have been given here.

Sticky Fingers is owned by Bill Wyman, not Keith Richards and is the same food as the Hard Rock.