Rusk
Also known as sausage rusk, breadcrumb, pinhead rusk, Semmelbrösel, panade
The dried unleavened biscuit that makes a British sausage a British sausage. It holds water and fat during cooking, and it is a texture ingredient rather than a cheap filler, whatever its reputation.
Rusk is a dried, unleavened wheat biscuit, baked without yeast and then broken to a specified grade, from a fine crumb to the coarse pinhead rusk most sausage makers use. It is not the same thing as breadcrumb, though breadcrumb is used similarly in continental recipes: bread is leavened and contains yeast, sugar and salt, and it behaves differently in the mix and on the shelf. Rusk is made specifically for this job.
The job is water. Rusk absorbs roughly three times its own weight in water and holds onto it through cooking. In a fresh British sausage that is the whole point: the rusk is hydrated before it goes in, and it keeps the sausage juicy rather than letting the water render out into the pan, which is why a well-made rusk sausage stays succulent where an all-meat one can dry and tighten. It also carries fat, holding melted fat in suspension rather than letting it pool, and it gives the characteristic soft, slightly open crumb of a Lincolnshire or a Cumberland.
The filler accusation is worth answering directly. Rusk is cheap and it does extend a mix, and it has certainly been used to pad a poor sausage. But a British sausage without it is not a better sausage; it is a different one, closer to a continental fresh sausage in texture, and the tradition that produced Lincolnshire, Cumberland and the rest is built on rusk deliberately. UK law sets minimum meat contents by name, and a sausage that meets them and uses rusk for texture is doing exactly what the style asks.
Hydration is the practical decision. Dry rusk added straight to a farce will pull water out of the meat and leave the mix stiff and the sausage dry, so the standard practice is to hydrate first, commonly at around two to three parts water to one of rusk by weight, and to fold the wet rusk through the meat. Recipes vary widely on the ratio and the correct one depends on the grade. Where a recipe gives rusk and water separately, do not assume they can be combined dry.
Rusk is wheat, so it is a declarable allergen containing gluten, and that is the single most important thing to record about it in a recipe. It has no place in a dry-cured product: it would bring water into something that has to lose it and give spoilage organisms a substrate. It belongs in fresh and cooked sausages.