Yeah but that would take effort.

With energy prices in the UK being what they are, it's only raw potatoes that are cheaper than bread. At least toast toasts quickly, so isn't that energy-intensive compared with boiling a pan of water.

You have to cook the potatoes or they're poisonous, and buying and running a hob or an oven is comparatively pretty expensive in the UK. I get your point though, if you're a bit wealthier, of course you'd be looking at potatoes or other foods - and it can get really ingredient cheap if you buy a sack of potatoes and mostly just eat potatoes :)

That's why toast with crisps is the more popular food.