In a food processor, combine the sugar, lime juice, Seedlip, and 1 bottle of beer, along with 3 large handfuls of broken ice. Blend until the ice begins to melt, then pour into a big jug and top with the remaining beer and lime segments.
On a saucer, strew the sea salt. After dipping the rim of a glass in water, dip it in salt.
Pour the lagerita into the cups and serve right away.