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
You might want to correct Warren Buffett since he recommends packing your lunch at least 3 days per week.