The wild organic Haggis, living on heather shoots on the upper reaches of Scots mountains, has now been hunted almost to extinction and is a protected species. Due to its unusual leg conformation, it cannot survive on flat ground. The two varieties are cross-fertile, but unable to mate on the hillside.
The mock haggis usually served now, is made of chopped sheeps' lungs and other internal organs, with oats, stuffed inside a boiled sheep's stomach, and boiled. It is technically a sausage, though often referred to as a pudding.
You may find the Tesco one is made with bits of cow, which is not correct.
Faggots are a sort of meatball containing pigs' livers and minced skin, with crushed bread, which is different.
If you buy a pack of Brains faggots, they do not contain brains, but are two-thirds a sauce made primarily of water and lard, and one third faggots. The meat proportion is negligible at 7% of the pack contents.
Ingredients
West Country Sauce (62%) contains Water, Lard, Wheat Flour, Modified Maize Starch, Tomato Puree, Salt, Colour (E150c), Yeast Extract, Sugar, Onion Flavour, Spice & Herb Extracts (Celery).
Pork Faggots (38%) contains Water, Rusks, Rehydrated Pork Rind, Pork Liver (15%), Onion, Pork (4%), Pork Fat, Wheat Flour, Salt, Sage, Spice Extracts.
("rind" means skin)