I think it is more about using up the leftovers, by throwing it all together in a frying pan and cooking it together until it is all hot. So each batch is slightly different. It would depend more on what the menu was from the meal or meals just before you make the B & S.
It is the left overs from a roast beef dinner. So as long as your roast beef dinner tastes like grandmas your bubbles and squeak, should be close. I know you have to have mashed potatoes and over cooked broccoli. I can ask my MiL about it. Meem and Papa are off the plane immigrants from England...