I`m not sure how far back you are going, but when BA and BM operated the route back in the late 1980`s BM had a 52% share of the BFS-LHR route, the aircraft were full but the yield was not as good as other routes.
Now these fares are quoted from BM timetable 31 Oct 1988-25 March 1989
**LHR-BFS**
SGL £72
KEY SGL £46
SBY SGL £43
3 DAY RTN £120
EXCURSION RTN £105
SUPER KEY RTN £83
**LHR-GLA/EDI**
SGL £76
KEY SGL £50
SBY SGL £48
3-DAY RTN £129
EXCURSION RTN £114
SUPER KEY RTN £77
Although not major difference in fares NI paxs seemed to pay less than Scottish paxs and then X the number of seats on a 737-300/400.