Work snacks would be things like a Mars bar from the vending machine at work.
If you are disciplined, this would be another easy way to save as individual chocolate bars, drinks, crisps from small shops, vending machines or even Tesco express cost 2 or 3 times the price of buying multipacks on offer in the supermarket or stuff from the pound shop, but you have to make it last all week, otherwise all you are doing is spending the same amount of money on more food, so not good for health or bank balance.
I think in this case, the OP is talking about disposable income as in luxuries/discretionary purchases after main bills and supermarket food is purchased.
A lot of this is 'nice to have' rather than a need - we all need to eat but obviously a packed lunch or meal cooked at home is substantially cheaper than buying prepared food. Also consider how you do your supermarket shopping. The difference in cost between wandering round Waitrose and filling your trolley with a load of 'that looks nice' items and organic blueberries compared to writing a list of cheaper meals from a less expensive supermarket can be hundreds of pounds a month.
A PP has nailed it in that the OP has a good disposable income and probably more than average, but it's still not enough to spend freely on everything all the time without thinking and as you have found, you appear to have spent hundreds of pounds a month on unidentifiables, which when you do work out what it's all going on, will probably horrify you if it's a lot of stuff that doesn't seem to be worth the cost to you.