Steven, I've driven over quite a bit of England (but not in the south) and Scotland. My first time driving in England was in 1996. I had a manual transmission and had no problems at all. Making the transition from right to left side driving took about 5 minutes at the most. As mentioned, the roundabouts can be tricky but the beauty of them is that you just keep driving around till you figure it out.🤣
When I was in Scotland earlier this year, however, I rented a car with an automatic transmission. The Scottish roads in the Highlands and on the islands can be a lot hairier than many of the English roads. There are many more curvy and narrow mountainous single track roads there, and I found it much easier not to have to worry about the transmission.
You don't say where you are driving to, but if it's anywhere in Cornwall or Devon, you might want to think about renting an automatic. I am traveling to England next year for 5 weeks. I've rented an automatic transmission for the Cornwall/Devon portion of it because of the higher amount of traffic there, the narrow roads and the high hedges that enclose them, and the very hilly roads along the coast.
However, I am renting a manual transmission for the time that I am in The Peak District, Yorkshire and Northumberland, since I am a bit more familiar with those roads, and there is less traffic there, and more open spaces.
I don't know about the rest of the south, so if you would be more specific about your destination, others here could help.
With regards to insurance, I always refuse the CDW and rely on my Chase Sapphire Preferred credit card for the damage portion. I have not had any damages accessed but in looking at the language of the benefits, everything is covered that would be covered by the rental company, so I don't like to pay for something I can get for free. 😊
Lastly, I agree with everyone who said to rent the smallest car you feel comfortable in. Believe me, you will not regret it - but you will regret trying to maneuver a larger SUV down those narrow single track roads, especially when you have to pull over to let a car pass.