For a while IATA tried to standardize fare codes and in the days of regulated fares picked P for "premium first" and J for "jazzed business", each of which were at a 10% price premium over regular F and C. So historically P/J indicated a superior product at a higher price than F/C. At some point this system fell apart, and fare codes are random (even F isn't F on all airlines!). As for what acronym J really is for, that is a mystery to me (though I do like a jazzed-up business class, but somehow that isn't the BA way, more like VS.