As someone who drives Berthoud Pass in Colorado on a regular basis (except when it's closed due to avalanches or avalanche control work, which happened a lot last winter), and who also managed to drive in some of Scotland this summer with nary a mishap, I'll throw out my two pence worth.
Following MC's advice above, I went to Ali's Cave in Edinburgh and got a green P for 2 pounds. It turns out there were 2 sheets of thin plastic with green P's in the package, and each had 2 thin magnetic strips to hold it to the boot (trunk) lid of the car. Good thing for having the second P -- the first one blew off during a storm one night on Skye, while the car was sitting still. It never came off while driving! It did give a little more peace of mind, letting the other drivers know I wasn't skilled driving from the right side of the car and on the left side of the road, though we still got 2 or 3 honks that week ;-) The main thing seems to be not staying TOO far to the left, as you don't want to drop off the pavement or hit a curb. The signage was good, and in English (plus Gaelic, too, some places), so navigating wasn't difficult in the hinterlands. Getting out of Stirling required asking directions. Driving in Glasgow or downtown Edinburgh could be a bigger challenge, though.
Compared to US roads (heck, even compared to US bike lanes) the roads were narrow. If the lane's wide enough for a car, you could argue they don't need to be any wider than that, but many drivers, including some in vehicles bearing license plates of countries where they drive on the right, had a hard time staying on their side of the center line. As we say, they were taking their half of the road out of the middle. We didn't have snow in August, but had a couple of really rainy days, and as with anywhere, just slow down, drive smoothly, and leave a greater distance between vehicles. I'd imagine there will be a lot less of the tourist traffic in December than we encountered (and contributed to) in August, so there's an advantage for you.
Regarding what Pam said, although I've taken the bus to Aspen many times, and it's not a bad way to go, Highway 82 has been widened in the past few years, making for a nicer drive. If you're going skiing at Sunlight or one of the four Aspen mountains, take the necessary precautions and get on the road and drive. It's snowed the past 2 days in the Colorado mountains so it's pretty right now and would make a nice visit, but Scotland in December would be great.