Potted shrimp is the paste-sandwich tradition with the protein left whole, and what defines it is the potting itself: the seal of spiced butter that both preserves the shrimp and binds the sandwich. Tiny brown shrimp, the small sweet Morecambe Bay catch peeled by hand, are warmed in butter spiced with mace, nutmeg, and a little cayenne, packed tight into a pot, and sealed under a clarified butter layer that sets hard on top. That butter cap is not a garnish or a serving flourish. It is the technique the whole thing is named for, an airtight seal that holds the shrimp under spiced fat, and when the set is spread on brown bread the butter is doing as much structural and flavour work as the shrimp.
The craft is in the spicing and the fat. The shrimp themselves are mild and sweet and would read as faint on bread alone, so the butter is loaded with warm spice, mace above all, in a quantity judged to season the shrimp through without tipping into harshness, and it is that spiced butter, melting back to softness at room temperature, that carries the flavour across the bread. The shrimp are kept whole rather than pounded smooth, which is the line that separates this from a fish paste: the slight resistance and the clean small bite of an intact brown shrimp is the texture, and beating it to a paste would throw away the point. Brown bread is the conventional carrier for the faint nuttiness that flatters sweet shellfish, and no extra butter is needed because the potting butter already supplies the fat and seals the crumb as it softens into it. A squeeze of lemon and a turn of pepper at the table is the only adjustment the set will take, applied so it lifts the sweetness without slackening the butter.
The variations are the rest of the potted and paste larder, each the same shelf-stable, butter-sealed logic met in a different protein. Crab paste and fish paste take the shellfish to the pounded, smooth end the whole shrimp deliberately avoids; gentleman's relish is the concentrated anchovy version used in the smallest possible quantity; the wider potted-meat and paste sandwiches share the seal-under-fat method with a cured meat instead. Each deserves its own article rather than being crowded in here.