A stereotypical "poor man's diet" has been bread and cheese, for example in victorian(?) london. But today, cheese isn't cheap, especially in comparison to bread. What changed?

The answer to this question is probably more related to sociology than history. The root of the question is how you are defining the concept of "expensive." If you measure the cost of something by comparing the cost of bread to the cost of cheese, then yes that gap has relatively increased. Mostly because bread is quite easily mass produced by machines whereas mechanized dairy farming has not created the same gains that mechanized bread making has. Imagine a factory that makes bread. To make tons and tons of bread, all you need to do is fill the hoppers with ingredients and the machine does the rest. It's infinitely scalable - if you want more bread just buy bigger machines and run the machines more. A huge advantage economically from the handmade breads of Victorian England. Whereas to scale a dairy farm, you need more animals, which means more touch time, which means more costs. You can't make a bigger cow, or a bigger udder very easily for example.

But I think the more interesting part of the question you're asking is "is cheese more expensive to a person today than it was to a person in Victorian England?" And the answer to that question is totally and unequivocally no, cheese is much cheaper today than it was in Victoria England. A commoner in that era would have spent much, much more of his income on staples like food and shelter than people do today. Examine this graph which shows the percentage of income spent on food for various countries in the world. More developed countries with better means of production spend much less on food, which frees their people to spend on other things that someone in Victorian England would have considered a luxury - like automobiles and various sets of clothes, etc. And so in real terms, both bread AND cheese are much, much cheaper than they were in Victorian England due to the magic of productivity. Even though you don't see it, this trend continues worldwide year by year. A reduction in % of income spent on food is an important metric in measuring if a country has pulled itself out of poverty.