As others have also pointed out, it is very cost effective to fly a dash and crew to Medavia, get the work done and return the A/C compared with other MROs, and STS is just another name on the former MAEL Hangar, with may of the same engineers (many also ex-Flybe) who've worked on these aircraft for decades.
STS also has the benefit that at BHX the planes are at home base/HQ so if there's a problem and it needed management attention, then it's just over the road.. & the upside to having a physical airframe nearby in a hangar in case of AOG elsewhere on the network are not to be sniffed at.