I checked Mrs. Beeton's to compare, but her "Bubble and Squeak" recipe doesn't call for any sort of catsup/ketchup. Nor does Hill's Breakfast Book
, or Sanderson's The Complete Cook
. In all cases, they agree that beef + cabbage is the essence of the dish (typically with onion). Cre-fydd's recipe
includes mashed potatoes and greens.
Closest I've found is Dodd's Housewife's Manual
which mentions ragout of mushroom, oysters, or onion as a nice addition to Bubble-and-Squeak. On the basis of that, I'd say to try mushroom catsup.