The Saab is operating on other SX routes from BRN too, not just BRN-BSL-LCY which is why its based at BRN. I can't imagine BRN-BSL-LCY being very popular, no matter how long the stop is, it's still a stop and adds time to the journey (in the eyes of the pax). Surely pax would be put off this and prefer a direct flight. On the other hand if you do some calculations I suppose the fact you arrive a LCY means its quicker overall.