I think there's a few things going on here and as Chil1234 says it's hard to be constructive without knowing more details.
The first to to note is that "poverty" is relative not absolute. "Poverty" is defined as earning less than 60% of the median-average wage.
So you sort the entire population of the country by what wage they earn, and the person right in the middle is earning the median wage.
You could give every single person below that media a pay rise to £1 less than the median wage and the median wage wouldn't move one penny.
So being "poor" in this country doesn't mean you've not got a house (though it might) or you can't eat (though again it might) but just that you earn less than 60% of what that middle person does.
Compared to the past and other countries many of those who are classified as "poor" are better off than they ever have been.
Then throw into the mix the plethora of cheap take aways that are now available (yes they are more expensive than cooking healthily but they're more affordable and more widespread than they ever have been).
So it's now more possible for many of the "poor" to choose an unhealthy diet than it ever has been in the past. And I think that's playing a big part in all of this.
Basically in my grandparent's time chips were a real treat. I'm sure they would have had them every night if they could, but the couldn't, they had no choice but to eat as cheaply as possible, and that meant healthily.
More people now have a choice, and unsurprisingly they're going for the tasty treats.
So unhealthy diets say far more about people's increased ability to afford what they want rather than education or that the previous generations were somehow wiser with their dietary choices.