Auto Europe is a US-based brokerage that offers lots of car rental options and has worked well for me and others. You can use the website to compare costs for different companies, vehicle types, and rental scenarios. Generally you don't incur significant dropoff charges within the same country. I'd assume Scotland and England, being in the UK, are the same "country" for rental car dropoff purposes, but I don't know this for sure. A UK-based poster will have a better answer.
Open-jaw flights, like Seattle to LHR + Edinburgh or Glasgow to Seattle, are often the best way to go. If one costs a little more than a round-trip, it's probably still cheaper than backtracking to where you started in Europe. And it certainly saves backtracking time, which ought to be worth something on a time-limited vacation. Round trips make sense if you're taking a circular route on land, but it often makes more sense to start your ground trip at point A and finish at point B.
As the other poster said, driving from London to Scotland is great if you want to stop along the way on your own schedule, but the train is much faster and easier. Or you might consider a train to, say, York, then getting the car for the scenic and interesting trip north from there. (No disrespect meant to places south of York.)