Anthropology

Salt was absolutely essential to life in northern latitudes, it was the only way to preserve things like cabbage so that people didn't develop scurvy over the long winters It also preserved meats and fish.

While there were domestic wines in Britain, the harvests were chancy, at best. Beer and ale were the preferred ways of sterilizing water so that it could be drunk safely. It was cheaper to make beer than it was to import wine.

I think Rome wanted a monopoly on the former and simply wanted to keep its army alive with the latter.

(Why yes, I've done deep dives into that region's history, Bronze Age through Mediaeval, that stuff is fascinating.