Other helpful websites
Each site listed below (and you'll find more with the help of Google) has content directly concerned with the actual act of driving, rather than issues such as the law, how to rent a vehicle, etc. You'll have to make up your own mind about the sites' respective utilities. I personally think the subject is too large and diverse for a website, which is one of the reasons I ended up writing a 100-page book. U.S. novelist Chris Orcutt (the first entry below) states: "I scoured the Internet for a good, clear, comprehensive guide for Americans to driving in the UK. What I ended up finding were a ton of small websites, blog entries, videos and PDFs, all offering different advice — some of it contradictory." I agree.
Take with a big grain of salt advice written by natives. They can't really understand the difficulties faced by N. Americans, and the advice is sometimes saccharine and with a (not-so-hidden) agenda. Finally, if you come upon a piece that was especially helpful to you, let me know, and it may end up here!
1. http://orcutt.net/weblog/2013/03/15/driving-in-the-uk-a-survival-guide-for-americans Entitled "Driving in the UK: A Survival Guide for Americans", this lengthy piece by Chris Orcutt aims to be a comprehensive guide. It's certainly far better than most, with useful links to videos. However, his calling the A82 road along Loch Lomond "sinuous and suicidally narrow" made me laugh. To quote, you ain't seen nuttin' yet, Chris, and there are far more difficult roads than this. It's all in vehicle positioning!
2. https://shershegoes.com/driving-in-ireland/ has 10 tips from a New Yorker for Americans driving in Ireland, and is notable for its gorgeous photography.
3. https://www.tripsavvy.com/safe-driving-in-ireland-1542976 has 12 safety tips about driving in Ireland.
4. https://www.tripadvisor.com/Travel-g186216-c9626/United-Kingdom:Driving.Tips.For.Visitors.html has a somewhat dense text with links to organizational websites of government and motoring organizations.
5. https://www.aransweatersdirect.com/blogs/blog/102911361-driving-tips-for-us-visitors-to-ireland A very unusual source for a guide, and although it's short, it has some good advice.
6. https://www.ricksteves.com/watch-read-listen/read/articles/driving-in-great-britain-and-ireland The redoubtable Rick Steves presents his short take on the topic.
7. https://europeupclose.com/article/driving-in-ireland-tips/ Ten elementary tips.
8. https://about-britain.com/driving.htm Very elementary and limited information, but a couple of informative photographs.
9. https://www.justlanded.com/english/United-Kingdom/Articles/Travel-Leisure/General-Road-Rules Worth reading for a slightly different take on things.
10. http://www.slowtrav.com/uk/instructions/driving.htm Elementary information, but some pretty photos.
11. https://www.tripsavvy.com/driving-in-the-uk-and-ireland-1662401 Again, very elementary information, but useful photos.
12. https://www.irishcarrentals.com/driving-in-ireland.php Irish car rentals claim their tiny guide is "The only guide you need to driving in Ireland" Yes, well ... I threw this one in last so that you compare it with the first.