Beautiful scenery, lovely people, but it's not like downtown anywhere, and it can be a very long way between one thing - gas station, shop, even a person - and the next. You'll have the road to yourself, so think about that.
I rented a bike in Switzerland this September directly from a dealer. Significantly less than through the rental companies which, it turns out, are simply brokers for the same dealers that you can rent from directly. Turns out that most large dealers in Germany and Switzerland rent just about anything they sell, so you might try this approach. Don't know about Scotland, but if you've got access to the world wide web, you know as much as I do.
There was a period of time when I was required to spend a couple of weeks at a time several times per year in Geneva, Switzerland. I longed for a motorcycle to ride on those glorious mountain roads. Here's what I worked out with a local motorcycle dealer. I would look over his used fleet and pick one I would like. I would agree to buy the machine for a certain price, and the dealer would set the down payment, which would equal what he was willing to rent it to me for. But, on the contract, he would grant me a refusal period, equal to how long I wanted the machine for anyhow. If I brought it back within that refusal period, I would get out of the contract, but lose the down payment (rent). This way, his insurance covered the machine if I pranged it while I had it. We did this off and on for 4 or 5 years. I also got to sample a wide variety of machines, some of which never made it to the US.
I did the Harley rental thing from Germany this summer and spent some time riding in Germany, Netherlands and Belgium. The bike was okay, the service was okay.
I learned to spend extra time planning routes, the roads here in the US are generally easy to follow with lots of accurate signs and when I go places here I have no problems. I had some troubles finding my way around in the countries I visited. GPS or extra planning probably would have taken care of that for me.
Go to you local bookstore, hit the periodicals and look for the Brit bike rags. They're expensive to buy but if you take a note pad and are willing to look the English have usually 7-10 bike shops that offer rental service. Pick the most convenient to your desired location. Also check on temp insurance.