I have clocked over 20 trips to London over the past 2 years and have easily eaten at 200+ restaurants. Here are a few of my "casual" favorites (inexpensive is a very relative term in London):
Da Mario (Italian) - 15 Gloucester Rd, South Kensington. It is impossible to order something bad on this menu - amazing pizza and fresh pasta. There is a good reason they've been in business for 45 years - and they are very family friendly if you are traveling with kids!
www.damario.co.uk
Kappacasein (Toasted Cheese Sandwiches & Raclette) - Borough Market, London. Open Thursdays 11am-5pm, Fridays 11am-6pm and Saturdays 9am-5pm ONLY! The most amazing gourmet grilled chesse sandwich you will ever have in your life...bring a friend because it is impossible to eat the whole thing by yourself.
www.kappacasein.com
Casa Brindisa (Spanish Tapas) - 7-9 Exhibition Road, South Kensington or either of their sister locations
Tierra Brindisa - 46 Broadwick Street, Soho &
Tapas Brindisa - 18-20 Southwark St, Borough Market. Very affordable and in-depth Spanish wine list and some of the best hand-carved, acorn-fed ham Iberico has to offer! Also not to be missed is the fried Monte Enebro goat's cheese with orange blossom honey...YUM!
www.brindisa.com
The Builders Arms (Traditional British Pub) - 1 Kensington Court Place, Kensington. Neighborhood gem that makes killer Bloody Marys and has an impressive selection of draught beer. The pub food is straight-forward and delicious. One of my favorite ways to remedy a hangover with a slow-paced Saturday or Sunday brunch!
www.thebuildersarmskensington.co.uk
Truc Vert (Honest, Humble French Cafe/Deli) - 42 North Audley Street, Mayfair. Menu changes daily including homemade quiche selections. OMG - they make their own butter here, 'nuff said!
www.trucvert.co.uk
Relais de Venise L’Entrecôte (French - No Menu, No Reservations, No Kidding!) - 120 Marylebone Lane, West End. There is no menu here, everyone is served the same thing a green salad with walnuts dressed with mustard vinaigrette followed by steak frites, served with a secret recipe sauce, in buzzy, convivial surroundings. The house red is excellent, and a bargain! Order dessert if you must. Plan on waiting in line outside up to an hour for a table...you're bound to meet some interesting people.
www.relaisdevenise.com
Y Ming (Chinese) - 35-36 Greek Street, Soho. Unpretentious, polite service...everytime. Peking Duck is to die for, order at least 4 hours ahead of time. I also love the double braised pork. I dare you to find another restaurant in London where you can order a deep-fried Mars bar for dessert! Book ahead...they are always packed. Great locale for pre-theater dining.
www.yming.co.uk
I could keep on posting for days...but these are a few sincere suggestions from a Southern Belle that finds herself on the other side of the pond quite frequently...ENJOY!
ATLgal