The rear brake callipers are only single piston which pushes the pad onto the disc on the piston side, the other pad is pushed onto the disc by the movement of the calliper on the slider pins. If the outboard sections of the discs are less worn down than the inboard (piston side) this would confirm sticking or seized pins and not seized callipers.
Pins cost around £10.00 each so £40.00 for the set, getting them out if seized starts with WD40 and ends with a slider hammer.