If Mary did buy her "last supper" from a Chandler's shop, you would have thought because of all the interest from the case that the shop Mary visited, someone would have come forward to say that Mary was in the shop at x time and she purchased fish & potatoes.
I never thought of it before, but that does seem odd. Like DRoy says, though, she may have bought it earlier, when the shops were crowded. It's quite possible too that she had reheated leftovers, maybe from the previous day, and eaten them alone in her room. Fish and potatoes would keep fairly well in a cool room, I think (jacket potatoes are good for several days, if the skin is unbroken) and she did have a window out.
I'd have to think the police went 'round asking at places that served fish and potatoes, even if it's not recorded. Surely those are the sorts of questions (What time did she buy it? Was she alone?) that the detectives would have wanted answered, especially after doubt about the time of death arose.
I think her last meal was fish and chips. It was an inexpensive and quick food commonly found in the East End at the time. From what I've read, chippies first began to open in the East End in the 1860's and quickly became a staple in the working class (or poorer) diet. Most people in the East End didn't actually cook in their homes. If you look at the population density and number of people per room, in many cases there's just nowhere to do it. Using chandler's shops and fast food vendors as their kitchen/larder was very common. Even someone like MJK who had a fireplace in their room didn't necessarily have the ability to cook on it. A small coal fired fireplace would not have the space or equipment to allow even the simple reheating of most food items and there are no cooking/eating items referenced in any of the reports I'm familiar with other than the tea kettle.