the working classes living in tenement blocks had little way of cooking staples such as lentils, oats and potatoes. So pre- prepared foods were the only way of feeding themselves.
This is ahistorical I'm afraid. Even the worst hovel would have had a fire in the grate because central heating didn't exist. People relied on fire for heating and cooking. Pre-prepared food didn’t really exist either and you wouldn’t have been able to afford it anyway.
“The habitual food of the individual working-man naturally varies according to his wages. The better-paid workers, especially those in whose families every member is able to earn something, have good food as long as this state of things lasts; meat daily and bacon and cheese for supper. Where wages are less, meat is used only two or three times a week, and the proportion of bread and potatoes increases. Descending gradually, we find the animal food reduced to a small piece of bacon cut up with the potatoes; lower still, even this disappears, and there remain only bread, cheese, porridge, and potatoes, until on the lowest round of the ladder, among the Irish, potatoes form the sole food, As an accompaniment, weak tea, with perhaps a little sugar, milk, or spirits, is universally drunk. Tea is regarded in England, and even in Ireland, as quite as indispensable as coffee in Germany, and where no tea is used, the bitterest poverty reigns.” (Engels, The Condition of the Working Class in England, 1845)
Yes, Britain industrialised first, but things like custard powder, commercial jam and ready-made biscuits only appear towards the end of the nineteenth century, and it was only after WW2 that tins and packets of food made up a substantial part of people's diets.
If you saw the TV show Back in Time for Dinner which was on a couple of years ago, most families were still cooking meat-and-potatoes-type meals from scratch until well into the 1980s.