My favourite shop is Mercadona, I find that there prices are good and all there own brands that I have tried have been very good.
I have a wheat intolerance and they have wheat free pasta and bread. Also can get vitamin tablets and other natural tablets at reasonable prices. It is difficult to get natural medicines at a reasonable price here.
Spanish bread is very sweet but they have a loaf in a clear and blue packet which is much more like British bread and is just over a euro for 28 slices.
Tenerife has also a few Iceland shops, but the prices are a bit high for some things.
I found something similar to smoked sausage in Alteza super market and it was under 2 euros a lot cheaper than the British one in Iceland, and tastes as good.