Sainsburys is good for most things and ours even has gluten free food in the freezer section (just a few things, their pizza is good). If you look carefully you can get beefburgers in the regular frozen section that are gluten free and cheaper - look for 100% beef and check the allergy information. However our Sainsburys don't sell xanthum gum (Tescos do) which is invaluable for making normal tasting gluten free cakes.
I am a terrible cook but make acceptable gluten free bread in the breadmaker with doves bread flour. It tastes more bread like if you use rice milk rather than cows milk. The shop bought gluten free bread is revolting. I have tried a different bread recipe and it was a miserable failure so I strongly recommend starting with Doves. Don't forget you can make cake in the breadmaker too.
Basics - pasta, bread and ordinary flour, xanthum gum, rice based cereal or cornflakes for breakfast, Sainsburys jaffa cakes and pizza for when you don't have time to cook, cornflour, rice, potatoes, oven chips with no wheat coating (quite a lot of chips/roast potatoes have a wheat coating), baked beans and cheese for quick baked potato with ....meals.
We love buckwheat pancakes but you'll probably have to go to a health food shop for buckwheat flour. If you see any Mrs Crimbles brownie type cakes are fantastic, but I've only seen them in Asda and a health food store. Asda also had buckwheat which you can grind to make flour (never tried that myself, the health food shop man said people do when he runs out of flour).
If you're very busy there are lots of other things that would save time but they tend to be pricey.