Hey, you can do it! I fell off the wagon myself. I went from 14st 8lbs 13st 8lbs. Had some life stress and fell off the wagon. Came back to SW at 15st (sob!). Am now 13 st 6lbs and counting! You can do it, you just have to make a definite decision and go for it!
As for food, in the long run SW food is cheaper. The food shop will probably cost more but will last waaaaaay longer than the type of shopping you described. I would say go to lidl or aldi. They have cheap EVERYTHING. Their pasta, rice, beans etc very cheap.Try to go shopping late, because then lots of stuff is reduced (I had bread for 20p yesterday!). Maybe get a big box of cereal for breakfast (cheap and lasts ages), use more eggs (Again, cheap and versatile), maybe try frozen fruit at the moment unless you find any offers (it's out of season, so fresh is expensive). Try not to by diet processed foods unless on offer, as they're usually quite expensive!
I think you need SW though. Keep eating like above and you'll get scurvy lol