Im not convinced they could make ABZ-STN-BRU. They would need to offer times for a business day return from both ABZ to STN and STN to BRU to be successful. And I doubt they could do that.
If say they left ABZ at 0630 the earliest they could depart STN for BRU would be about 0830 which would lead to a late arrival in BRU. For business passengers it would probably be quicker to take the Eurostar or even go from LHR