Pam's Greasy Spoon would typically have opened very early to serve cooked breakfasts to workmen and commuters. Short order cooking, air heavy with grease and smoke. Probably a quiet time during the morning when just a few retired or unemployed people, or housewives wanting a break during shopping, might come in for a cup of tea or milky coffee, maybe a bit of toast. Workers would throng back at lunchtime for more stodgy greasy food - typically sausages, beans, pies, eggs, chips with everything, often one token less greasy dish as special of the day - e.g. shepherd's pie, liver and bacon, stew.
What's happened in recent years is that coffee shops have sprung up everywhere and taken away a lot of that casual trade during the lull between breakfast and lunch, and often at other times too. Pre-packaged sandwiches and so on (lunch deals) are readily available everywhere and fewer people take a proper break at lunchtime, so the lunchtime trade is probably down. Traditional British food has a lot of competition from Indian, Chinese and other Asian cuisines, Italian food, sushi, burgers, chicken shops and for some people healthier options like vegan/vegetarian/juice bars. No small place can cook a huge menu to order. If people want variety, the proprietor pretty much has to buy in ready-made food, I'd imagine.
It's all happened quite fast. We've been living in our area (SE London) for nearly 40 years. When we first moved here greasy spoons were the only cafes for miles around. Pubs didn't really do food. We had an old-fashioned Italian restaurant which closed shortly after we arrived (unconnected!) and then a winebar opened, which offered food, but only a very short, limited menu. There was a very good Indian restaurant about a mile away. Quite a few takeaways of one kind or another. That was it, within walking distance. It's very different now.