For bulk buys of staples like rice, pasta etc, check out the local Asian or other ethnic corner shop. 5 or 10kg bags of rice can be ridiculously cheap. Ditto beans, lentils, tinned chickpeas etc for bulking out those casseroles.
Use your freezer. When you get a carrier bag of toms for a quid, use them for a basic sauce and freeze the portions.
A slow cooker is a good investment. Mine is the basic Tesco one, cost a tenner.