Yes, Duo was a great airline while it lasted. It was based in BHX and took over some of the short-haul routes which had been dropped by BA.
At one time, BA operated a Eurohub at BHX which sadly did not survive.
Niche carriers like Bmi Regional fill the gap between the big hub airlines and the LCCs. They offer convenience (for regional travellers) but this convenience comes at a price.