After posting a narrowboat trip report, I got some driving questions which I'm going to answer here.
For background, we are U.S. citizens and our only foreign driving experience was Canada (same as U.S. but in kpr) and Spain. Spanish roads through villages were narrower than we were used to, but the speed limits were slow enough to make this feel safe. Also, if the road only fit one car it was either one way or had a traffic light making it first one way than the other. We found roundabouts easy, with the added advantage that if you don't know where to turn you could just go round about again.
Wales was different. Driving on the "wrong" side of the road was surprisingly easy for my husband ( I did all the Spain driving easily, but would have failed British driving as the right left shift never clicked for me and I was only a passanger ). What undid him, and might have been easier for me on the right, was the speed at which the very narrow roads were posted. Roads where a compact had 1.5 to 2 feet of clearance to divide between a slate wall or hedge and oncoming traffic were posted at 60 mph. This was hard to stomach even in a taxi van where someone else was driving.
Dual Carriage Ways were easy, just like freeways but with roundabouts. Rural roads frightened us. In villages streets that only allowed one car at a time were a two way free for all. This was made tenable only by the politeness of most of the drivers.
I'm not saying we won't drive in the UK again, we probably will, but it was 300% more stressful than driving in Spain. And we decided where to daytrip based on potential stress.
I would add that learning to pilot a narrowboat was much easier than driving.