I drove for a week to Suffolk, Bath, and Oxford from our B&B near Earl’s Court. In a manual transmission VW Golf. Lovely car, and I had a great time.
A roads and motorways are similar to driving in America, as far as the road quality (particularly single or double digit A roads). The difficulty is getting started on a trip; you tend to veer to the right. Once I got onto the road I was fine.
B roads are best avoided if you’re trying to get somewhere in a hurry and narrow roads make you nervous.
Roundabouts are fairly easy to navigate; I lived near Boston and dealt with them all the time. The key is not to freak out when you approach one. As someone mentioned upthread it’s perfectly fine to go around twice… or thrice.
The biggest adjustment is with signage. First, UK roads have incredible signage, on the motorways at least. I think I was on the M25 orbital when I saw a “Get in lane” sign… about five miles before the exit. In America you might get a half-mile’s notice. But Yank roads typically give you directions - Hwy 183 North, Interstate 10 West, etc. In the UK it’s just “M40.” So you need to know where you’re going, and the towns along the way.
Motorways have many fewer exits than interstates do, so make sure you’re headed the right way before you get on the slip road.
I guess in this GPS-savvy world that might be a minor concern.