The cost of the Shuttle backup was sometimes ameliorated by a reciprocal agreement with British Midland to take excess BA passengers. BA in turn carried BMA pax when the destination weather was below limits for the Midland DC9s that were only CAT II capable. This happened to me two or three times between LHR and GLA when I was tranferred onto a CAT III equipped Trident or 757.
The Super Shuttle's improved in-flight service was I believe a direct response to BMA's excellent 'Diamond Service' catering.