Most, if not all of operators in BRS, and probably around the country for that matter, have agreements with handling agents, more so BRS + Aviance because of the diversions seen during the winter.
I don't think its top of the priority list who the handling agent is. Pilots have diversion points and this is determined by many factors, weather, convenience to pax, ability to keep some kind of a schedule, however rarely determined by handling agent!
ANY handling agent, regardless of name, will only cope with the staff numbers present at the time and the current operating schedule of base airline flights.
Yes servisair handled most BRS diversions, but then Aviance handle most LHR diversions, as it seemed last year BA had priority.
The only thing the airlines want is someone (anyone!) to meet the aircraft, assist the pax and crew, and get back on schedule!