Thanks for the infos.
At this point, I don't know if it'll be cheaper to buy invididual tickets, or get a pass (either Interrail or BritRail.)I'm waiting for a couple of travel guides for England and Scotland so I know what parts to visit.
Glasgow - Edinburgh is only about 50km. I can do this in about 3h, but I'll check if the area is worth it or better take the train.