Bloater paste is the strongest thing on the paste shelf, and the reason is in the fish before it ever reaches the jar. A bloater is a whole herring, ungutted, lightly salted and cold-smoked so it stays soft and gamey rather than drying hard, an East Anglian cure built around the oil staying in the fish. Pound that into a paste and the salt concentrates alongside a deep, slightly bitter, marine smokiness that no beef or ham paste comes near. This is the defining fact: bloater paste is not a fish version of meat paste, it is a different and louder thing that happens to share a jar.
The craft is restraint forced on you by the fish. The paste is spread thinner than any meat paste because its flavour climbs faster and finishes longer, and butter underneath does heavy work, both carrying the salt across the slice and softening an oiliness that would otherwise coat the mouth. The bread is soft and entirely plain on purpose: the paste is the whole statement and any assertive crust would only argue with a filling that has no texture and a great deal of flavour. Cut thin and pressed cold, a little goes a long way and is meant to, which is the entire point of a potted fish in a country that learned to make a meal out of the cheapest catch in the net.
Bloater paste sits at the strong end of a graded shelf. Beef and ham pastes are milder and lean on their cures; crab and salmon pastes carry the potted idea into sweeter fish; the made up sandwich is its own consideration, the same paste read from the bread rather than the jar. Each deserves its own article rather than being crowded in here.