And with the 'luxury' items, only buy them every 2 or three weeks not every week.
Buy cheapo value biscuits.
Look at your list and see where there are cheaper alternatives - the choc chips example above is a good one.....a cheapo choc bar instead will do just as well.
When you use meat in cooking, cook stews etc and use less meat and add more cheap veg. It is less obvious in a stew.
And is there any sense of rationing what your teenagers eat or can they riad the fridge anytime they like? I would make clear what is a free for all anytime, and which things need to be left. And if they eat all of the snacks etc very quickly,don't replace until the following week.
Yes to making clear what is a lunch item and what is for a main meal, so they aren't effectively having 2 main meals a day. Yes I expect they eat loadss, but do t need 2 main meals a day.
How about cooking up a huge load of something like pasta sauce with veggies which will do a couple of days of meals?
How about value soups as a cheap lunch if they are at home?
The thing is, you have to really want to do it. Rather than making excuses and saying 'we must have X,Y and Z' you have to be willing to try without.
Another way to do it would to go the supermarket with say £80 in cash and no cards. Shop with a calculator. You simply won't be able to overspend and once you go over £80 you will put back what you don't really need, or swap it for cheaper goods.
If you do this, you may find there is less choice that week or less treats. There maybe a bit of moaning but you won't starve.
And finally, tell the whole family a cut-back is going on. Tell them you need them on board to help and point out something positive as a result...ie there will be more money for X. How about asking each family member to nominate just 1 thing from the weekly list that they could do without.
I think its hard if you are trying to buy for a family and cut back without anyone else being involved. They need to buy into the idea.