They used to be sold by pie-men, who bought the meat fresh from Smithfield and the eels fresh from Billingsgate. Mincing meat of course stretches it further, and you can add bits and pieces to it - meat cuttings are all protein after all.
Buying from a pie-man was the equivalent of fast food
My grandparents were too young for the pie-man, but remember the ice-cart and the muffin man.