"Making your lunch at the office and not bringing Advil from home? I know generationally wealthy investment bankers who do these things"
Generally they don't like to waste money. That's part of how they stay rich for generations (as opposed to someone who hits the lottery and the next generation is basically back to where they started in many cases).
You think someone with generational wealth would stop to even consider the cost of advil or a lunch in FiDi? They don't. This little bit of mythology has always been amusing to me. They may make their own lunch out of habit or simply because they have a preference or whatever--but cost isn't a reason for people of that much wealth.
They don't like wasting money, but what they consider "waste" is not the same thing as what someone who isn't wealthy would consider waste.
Fun anecdata from my own life:
I grew up under varying amounts of poverty and now I make Big TechCo money. As I have gotten a lot better about shedding a certain kind of survival-hoarding-mindset "I don't need to keep <this half-broken thing> because I can get a new one when I need one again", I have found myself developing new neuroses about "waste" of things that my old self would have considered insignificant: (personal) office supplies, succulent leaves that have fallen off but that I can propagate, 5-year-old magazines I haven't read yet. It's like I maintained some amount of the anxiety but in the absence of the pressure to direct it usefully, it's gone cuckoo.
If you like going to the cafeteria or out to lunch, by all means you should do so if you can afford it. But it's also entirely reasonable not to spend money out of habit. For example, while I may do so when traveling, I have zero interest in getting a daily Starbucks fix.
(I often drive by a local Starbucks which usually has a line of cars around the block getting to go. I have to wonder who these people are who want to wait for 20 minutes or whatever to get their $5 latte at all hours of the day.)
If you make £100,000 then an hour of your time is worth ~£48.
You can get lunch for £10 or less, and cooking at home doesn't cost £0, so unless you are replacing a fancy two course lunch with a made at home sandwith, I dont see you saving over £2k a year.
Warren Buffet is cool and all, but he is not the world's lunch expert
Generally they don't like to waste money. That's part of how they stay rich for generations (as opposed to someone who hits the lottery and the next generation is basically back to where they started in many cases).