Although you are booking through bmi website it could be an SK fare that is being used. Y is an IATA generic class for Economy however individual airlines sometimes allow Y fares to be seated in a seperate zone (as BD do with flex economy) however that is an in-house procedure for BD only so the international portion may be governning the fare in this case. Someone in the know would need to look at history of your booking and see if the "Y" request was rejected at some stage.