The standard rear discs are 280mm IIRC. Vocky had a larger set of rear discs and calipers a while ago from another vehicle but i'm not sure if he's still got them?
As for changing the caliper on the front...........you don't need too. When upgrading tp 308mm front discs you use the existing caliper and pads but you do need to buy the larger caliper bracket that extends the reach thus covering the new disc radius braking surface. These brackets are available from LMF or Vauxhall.