I dont` actually fly for them, but I know some folks that do! So I will try and answer while we wait for someone that really knows to reply!
Not much Atlas flying actually operates from STN. I think GSS does some BA flying from there but that is a seperate company. Most of the flying originates from Amsterdam, Frankfurt, Dubai or anywhere else on the globe and crews are positioned on commercial passenger airlines from STN to the trip origin. I assume once on the road you could end up anywhere on planet earth as the company does a mixture of contract flying for the likes of Alitalia, Emirates, Qantas etc as well as military and commercial charters.
Disclaimer; This could be a load of tosh but hopefully an Atlas crew member will eventually put us straight! Cheers.