When we were traveling in November, we knew early on that there would be days we would want a car to take in the gorgeous scenery in Ireland and Scotland, so we planned a few days of our trip to rent a car and make the drives ourself. For Nick and I, it has been our experience in the past that it works out better if I drive and Nick navigates, so that's what we did and I got to drive all over the Northern Irish and Scottish countrysides. After driving 3 out of 7 days on the trip, I definitely got a good hang of driving abroad and it's not as nerve-wracking experience as I had anticipated.
Over the course of the few days we drove, including both in cities (Belfast and Edinburgh) and countryside, I came up with a few tips that might help you get over the challenges that might arise.
In terms of driving in the UK, lanes in the country are very small and often don't have any space between the oncoming traffic, not to mention the non-existent space on the side of the lane off the road. Especially when trucks are coming your way, it's very important to be driving safely and in your lane, to make sure you don't side-swipe someone. Once I got the hang of it, it got easier, but at first, the narrow lanes made me really nervous.
Another important factor when renting a car in the UK is knowing what kind of gas you'll be putting in the car. In Northern Ireland we had a VW Golf that took unleaded fuel, in Scotland we had the same exact car but in a Diesel version. When we filled up with gas before returning the car, both kinds of fuel are very clearly marked at the stations, just be sure to know which you're pumping in before you start.
I also found that people really only pass when needed. In the States, many times, the passing lane ends up being just another lane filled with people, in the UK, most of the time, people only used the passing lane when they were actually passing and then got right back into the slower lane afterwards. This really helped with the flow of traffic.
Roundabouts are another huge part of driving in the UK. At first, they really threw me for a loop (pun intended), but after a few of them, I felt more confident. The trick is to know ahead of time which lane you need to be in. For me, usually I was taking one of the first exits, so I tried to stay on the inside lane, though signage is actually pretty good at most roundabouts we encountered so just be aware and be ready to exit at any time.
One thing to note, when you are coming in and out of small villages, you'll see signs at the beginning and end of the town noting when to slow down and when you can speed up again. At these signs are also speed cameras, so be aware of what speed you're going at all times.