There’s plenty of videos on YouTube that’ll give you an idea. I’d suggest not driving on arrival day if you’re coming on a long haul flight, your brain will already be scrambled from the jet lag.
We rented a car on the outskirts of London and then headed away from the city. I was focused initially and it wasn’t much of a problem. Then I stopped at a gas station and when I went to pull out onto the country road, old habits kicked in, my attention was the wrong direction, pulled out to cross a lane and almost got hit by an approaching car.
Pretty much right off the bat in South Africa the first time, and then it was easy for subsequent South Africa and New Zealand trips.
I’ve driven in NZ and Scotland. Both times, I was able to drive pretty easily after a few hours (though driving always took more thinking, I was never on autopilot the way I sometimes am here - which is a good thing). Both times, I’ve started my rental with a longish drive so that I could get used to the car.
I got automatics. Here, I drive a stick. On vacation, I didn’t want to deal with shifting with the wrong side, and also I’ve read too many horror stories about Americans being given cars with faulty transmissions and having to pay for a new one. (Easier just to pay a tiny bit extra so that won’t be my problem).
After about a day, the biggest problem is that I would still walk up to the “driver’s side” of the car, only to find that the steering wheel wasn’t there.
If you really want to prepare ahead of time, having a look at The Highway Code probably isn’t a bad idea, at least for the road signs and road markings.
The hardest part for me was getting a good feel for the left hand side of the car relative to the side of the road. For the first few days I had a tendency to drift to the left in the lane and had many time where I’d brush against vegetation on the left side of the road. (After three weeks of driving on the left, I returned home and for a few days had to fight an urge to drift to the right side of my lane).
The biggest mistake was choosing where to start out. Instead of renting in London, I took the train out to Plymouth (we were going to drive from Cornwall to Scotland). I figured starting out in the country would give me time to get acclimated to driving on the left without the pressure of traffic. Boy was I wrong.
Just about every non-motorway in Cornwall is maybe one and a half lanes wide, no shoulders, sunken, bordered on either side by hedgerows or stone walls, with all the locals barrelling along at 60+ MPH. Spent the whole time in a non-stop game of chicken. Once we got out of Cornwall things were better until Scotland, but by then I was in practice.
We stayed in B&Bs most of the trip. At breakfast pretty much every Brit would burst out laughing when I said that I thought Cornwall was a good place to learn to drive on the left.
I’ve been out for most of the day, but no-one told me Scotland left Britain. It wouldn’t entirely surprise me, but I expected more of a build up… 
We’re actually just not very consistent about which side is the wipers and which is the lights- both UK and NZ- it just depends on your car model. My parents have one car with each, so they’re constantly turning the wrong one on. My last 4 cars have alternated sides as well.
I’ve not driven in the US, but be prepared for some very narrow winding roads. Oh, and the speed limit is a limit not a minimum (except on motorways, where it is pretty widely ignored unless there’s a reduced limit). Especially in the countryside, they’re not an indication of the speed you should go; roads here can technically have a 60mph limit even though it’d be suicide to go much over 20.
My main experience was in 1964 when automatic was not an option (at least not in a Mini-Minor). I spent a week in London without a car, then rented one for a month driving in England and Scotland. By a week I was sort of used to crossing streets and getting used to driving on the left was a snap. I had once test-drove an Isetta with left hand shift and I knew that was no problem and it wasn’t. If anything the left hand shift kept me centered on the change of sides. The worst was on one lane roads in Scotland where you had to remember that you pulled over to the left when meeting on oncoming car.
Which reminds of the Norwegian I knew who was a student in Oslo with family in Kirkenes, a couple hundred meters from the Russian border. Driving home late at night, the one lane road went in and out of Sweden (which still had left hand driving) several times with no signage and when he met an oncoming vehicle he had to guess whether to swerve to the left or to the right.
Later on, I drove a left hand drive car in London, but only to get from London where I had had the car delivered to the ferry to Europe.
Done it in the Caymans. The biggest issue I found was sometimes turning into the wrong lane at intersections - that stopped when I forced the habit of reminding myself at each one where I was going, rather than overconfidently pretend “I got this”. I never did stop signaling turns with the wipers, though. 
I had the car available to me in Edinburgh for 2-3 days and was totally intimidated; then we left the city and I drove for miles on regional highways headed north. Something “clicked” and all of a sudden I had a visceral sense of where the left edges of the car I was driving were and quit freaking out. Right turns took a bit of conscious thinking but they have rotaries (traffic circles) which are easier.
A friend of mine from Las Vegas visited me here in England a few years ago.
I had arranged for her to hire a car and the hire company kindly agreed to give her a supervised 20 minute trip for practice.
She soon mastered driving on the left, roundabouts and general traffic flow on our narrower roads.
After that she drove everywhere, including several days visiting Scottish Castles.
I debated whether I should write “England”, but figured it would be clear enough what a Yank meant.
One thing I’d like to recommend, since it works well for me when I’m planning a trip to the UK. If you know where you’ll be driving, rehearse your routes on Googlemap’s Street View. It will help you to get familiar with what the landscape looks like and the kind of roads and roundabouts you’ll be facing.
There aren’t any motorways in Cornwall… 
There’s the A30, which has 2 and 3 lane stretches, but the rules are slightly different. Mostly not relevant to tourists, unless you were planning on cycling or bringing a horsedrawn vehicle, but still; this is the Dope and I’ll be picky if I want to.
But yeah, Cornish drivers are terrifying. I moved from central Bristol, expecting a return to nice relaxed country roads, like I grew up with in the Lake District. Nope. They all drive like maniacs and only about 1 in 10 seems to understand the concept of a roundabout.
I did fine in one day in Australia; the only time I hesitated was a multi-lane interchange.
A 13 year old Celica stick. Fun in the Christmastime sun!
Actually, this was a problem with Scotland, the one lane roads with turn outs on cliff sides. I couldn’t do those quickly - backing up a hill with a steep drop off on one side when the car isn’t where I instinctively think it should be (behind me to the left rather than behind me to the right) was nerve wracking. (Especially since the oncoming car was inching toward me the whole time.)
But it wasn’t a problem when I saw them coming and could turn out and wait for the cars to go by.
Yes, that was my experience in England.
Tell me about it. There’s nothing quite like driving a narrow track barely wide enough for one car, and then having it gradually peter out on the edge of a cliff (an experience in (yes) Scotland). :eek::smack:
When I moved from the US to Australia, I was pretty much driving straight away with no trouble.
I found it interesting that when I went to get a driver’s license here, they didn’t ask for a driving test - they just wanted my old (Florida) license, and an explanation of what the codes meant.
Most of the potential problems you’ll encounter, and any tips, have already been covered. By far, the biggest pain is the turn signal/wiper mixup mentioned above; it’s a very muscle-memory kind of action. On the cars I’ve driven, the indicator stalk is on the drivers-door side of the column, so that switches depending on which country you’re in. I gather some manufacturers don’t follow this convention, however.
You’ll be fine. You’ll get the hang of it surprisingly fast.
Hopefully you’re comfortable with a manual transmission. You’ll pay a premium for an automatic in the UK, or in Ireland.
Getting used to crossing streets on foot actually takes much longer. In fact, I never really got used to it at all.
Funny – I’ve spent a fair amount of time in the UK and Ireland on a motorcyle. That takes no getting used to at all. Even the roundabouts are fine on a bike.
{Emphasis mine.}
…which reminds me. Once you get used to roundabouts, you will love them. They are so much better than 4-way stops!