The Y booking class is also used for companies which have a Y only domestic travel policy and cannot tell the difference between domestic J and international J. I don't think there is a price difference between J and Y, but there a no doubt corporate or TA discounts that can be applied as the fare I often pay via our work travel agent LHR-MAN-LHR in J is a fair bit less than the fare of £498 which books in to Y advertised on the BA website.