The Atlas, Polar and Evergreen flights using go over the Atlantic on GTI, PAC and EIA callsigns respectively. After Prestwick they then change onto a RCH callsign and usually go to Incirlik or Kuwait with some other destinations thrown in.
I was a Prestwick last Sunday afternoon and if my memory serves me correct there were 8 747,s on the ground, 4 on schedules and 4 on the RCH flights. 2 Polar, 2 GSS, 1 Cargolux, 1 Evergreen and 2 Atlas. Quite impressive.