I don't know anything about the Britrail pass but presumably it covers all the train companies. I would suggest you try here as well - http://www.thetrainline.com/ - you can book tickets there and just buy what you actually need and it may also give you a comparison with the Britrail costs.
Trains don't go everywhere but if you just want to go to Glasgow, Stirling, Perth, Dundee, Aberdeen or Inverness then you can from Edinburgh. There are also plenty of bus routes - http://www.stagecoachbus.com/region_2.html -from Edinburgh for the places the train doesn't go. And if that doesn't get you where you want to go then, as Jock says, hire a car.