Don’t.
If you are not from GB or another drive-on-left country, think twice before hiring (renting) a car. Accidents are frequent for visitors. If you nonetheless want or need to drive, familiarize yourself with the Highway Code and read these suggestions by Rick Steves.
Driving on the other side of the road is not the only challenge, but let me first report . . .
The good news about driving in the UK
British drivers are on the whole more competent than US drivers and are much less likely to hotdog or drive aggressively, a fact that shows in traffic statistics. The rate of fatal accidents per million population, for example, is four times lower in the UK than the US.
No doubt this is principally due to the greater difficulty in obtaining a license in the UK. There, you need to demonstrate more than the possession of a pulse and the ability to read an eye chart. Of course, they do have accidents. Some UK drivers are speed demons, tailgaters, clueless, or rude, and from time to time you will encounter the feckless driver who believes 30 is an acceptable speed on a 70 mph motorway. But you’ll encounter these types far less frequently than in my native land.
The bad news about driving in the UK
The motorways (like interstate highways in the US) and the dual carriageway A roads are excellent. But apart from those, British roads are terrifying. Even many A roads, which are well paved and maintained, clearly signed and marked, have two-way traffic in roads whose width is narrower than a two-car garage in the US. Literally. Width per vehicle in a US garage is supposed to be 11 ft minimum, and roads are much wider. You’re lucky to get 10 ft per lane on British roads. And the roads almost always lack shoulders and are edged with stone walls or hedges or hedges covering stone walls. I should mention the roads curve a lot. And the posted speed is often 60. And British drivers drive fast.
Then there are B roads and lesser thoroughfares, abundant in the beautiful countryside you so wish to see, whose single lane accommodates two-way traffic. I regret not taking pictures of signs that showed a one-lane road narrowing and growing curvier, but I was too busy praying.
In fact, the UK government reports that “60% of fatalities occur on country roads.” This is due to their narrowness, the walls and high hedges, blind corners, and the speed.
Some places are worse than others. Cornwall, for instance, has the worst drivers in the country, as well as absurdly narrow roads.
US visitors also need to acquire skills for roundabouts, which are good for relative efficiency and greater safety than 4-way stops (much less danger of being t-boned), but confusing to those with little experience of them–particularly when exiting into the correct lane.
Finally, though most fatalities occur on country roads, most accidents happen in cities. Given the generally high quality of public transportation in and between British cities, you really don’t need a car, especially not in London—where you’ll be charged a daily fee to drive in the city center.
Driving safely in the UK is far from impossible for foreign visitors, but the challenges have to be anticipated.
(Of course, you might do as I did: marry a Brit and let them drive. Problem solved.)