I know what you mean. I hate to sound all 50's housewife, but I do wish we had had lessons in personal finance and budgeting, and cooking for a family.
My mum has always worked full time since I can remember so we always had a cleaner and a hot school meal and it was up to my sister and I to sort out an evening meal - be it a pot noodle, hot dogs or sandwiches .
We had no role model when it came to looking after a family whatsoever, and I don't think it's a coincidence that both my sister and I have chosen to be SAHM's.
I do think f&v is expensive, but then again your household are vegetarians so I would expect your main spending to be on f&v.
I find the best way to stretch the food budget is to batch cook and freeze. I also balance the budget by having a few cheap meals like boiled egg and toast, beans on toast, jp's, and homemade soup.
Also initially spending money on storecupboard staples can help vavavoom a bland meal into something tasty and a bit special.
You also can't beat making things from scratch to help safe you £'s. It can be time consuming at first but once you get into it it gets easier and quicker. When I used to make a cake it would take me AGES. Then the more times I made it the faster and better I got.
But, the best way is to stop 'nipping' into M&S and Waitrose. I used to live in town too and know how easy it is to spend spend spend when it's easy and convenient.