We took the train from London and rented a car in Salisbury, drove it through Avebury and all the way up to Scotland. I have vivid memories of that initial car rental experience...
We were waiting in line to pick up or car at Salisbury (don't recall the agency, probably Hertz or Avis, does not matter). Another American couple was ahead of us. After they finished with the paperwork, they went out to the lot, hopped in their car, fired it up, and departed. I sat down at the rental desk, started handing over my paperwork. Suddenly there were loud tire-screeches and a very loud CRASH sound. We all jumped up, ran outside. The previous couple had driven out of the rental car lot, turned right, accelerated, and immediately had a head-on collision with an oncoming car coming around the corner, right outside the rental car lot fence. Nobody appeared hurt, but both cars looked totaled.
Of course, the American driving away was driving on the right side of the road, as we are in the habit of doing. He crashed right into the local person, correctly driving on the left side, who had just rounded the corner. Oops. Several (visibly angry) staff members from the rental agency ran out the gate to the accident scene and started dealing with it. The counter person stomped back into the office, I followed, we sat down, the guy shot me quite a look. I burst out, "don't worry, I won't do THAT! I know which side of the road to drive on and I promise I'll drive carefully, really." We concluded our business, I drove away v-e-r-y slowly and carefully (having to drive around the tow-truck that was hooking up the crashed cars in the street).
The driving was mostly uneventful after that.
That said, expect the first few minutes to be high anxiety (this is always the case for me, even though I've driven in many countries before, including more than a few times in wrong-side countries previously). I just always assume the first 15 minutes will be the most challenging. You need to bring your A Game to this driving task. No distractions. Stay focused 100% on just the driving, and nothing else. Your front-seat passenger must be your helper/navigator/co-pilot/support system, they need to be "on task" as much as you (all the time), and they need to take this job seriously (as serious as a head-on collision at a combined velocity of 100 mph or more). If there are back-seat passengers, I read them the Riot Act first, make it very clear that the driver is not to be distracted. Your helper should remind you (frequently) to "drive on the LEFT." I ask my copilot to verbally call that out every single time the car comes to a stop, especially every time you turn on to a new road (my brain tends to automatically "reset" to right-side driving after coming to stop and turning onto a new road). I also ask them to regularly help "spot me" and estimate my distance from the "edge" to ensure I'm centered left/right in my lane, whether on a country road (careful about how close you get to the edge, the edge may be overgrown bushes, a stone wall, or a raised curb), or even on a milti-lane highway.
One car we rented had a sticker on the inside windshield with a helpful reminder to DRIVE ON THE LEFT (the linked photo is from Ireland, but I think it's useful anywhere driving is on the left). That's useful, but my driving habits are so automatic, I like having more difficult-to-ignore reminders.
You will find some narrow roads, but most are great. Be prepared for the roundabouts (your helper will need to help call things for you in them).
Be extra careful every time you begin driving for the day, your brain may reset overnight. Our deeply-ingrained habits and muscle memory you have to overcome and (temporarily) unlearn. The first time you get behind the wheel after you get home, be careful about your habits, too.
Hope that helps. Good luck!