Great advice Chris.
In 2013 after a cruise, we rented a car from Harwich in East Anglia (East Coast) and had three days of driving in England. What made driving fairly easy was having an automatic transmission and a navigation system. Also, my wife kept reminding me "stay on the left."
I adjusted well, but didn't try to get into a hurry, and concentrated on driving, not drinking or eating while driving.
The double roundabouts were the biggest issues. We have roundabouts here in Georgia, but no double ones.
One big mistake that we made was driving on a Friday afternoon on the M25, orbital around London. It was awful, a real parking lot.
We did visit Windsor Castle, Stonehenge, Salisbury and Stone Circle during our drive, which went very well, since we were on the West side of the M25.
We are planning a four week driving tour of England and Wales next October and going to visit a lot of wonderful places (skipping London and the SE of England this time). The British countryside is amazing. Can't wait for our trip.