Yeah, food and drinks with a high sugar content will cause a drop in energy after a couple of hours or so. It's important to find out what form of sustenance works best for you. If the English breakfast and healthy bars work, that's great. (But I can't help asking: what exactly is an English breakfast?)
My own prescription for OTB game days is high protein, low carb. I take hard boiled eggs and canned fish for snacks between games. That's not expensive, and it keeps the brain functioning at a normal level throughout.
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I am just going to cancel an otb tourney hotel booking in blackpool because I dont have the money to go
I used to take coka cola cans to drink during games and I used to have wotsits or quavers and mars bars/snickers to provide me with food during an event while getting the hotel full cooked English breakfast and pile it on in the mornings. This strategy was both cheap and effective but my chess coach said it was bad, he said you get a high from all that energy food then the brain hits a low, so I started buying healthy breakfast bars, and water bottles, my thinking improved immeasurably while still having the cooked English breakfasts.