Have I got it correct? For the first 12 months you can drive on the USA license, and that time starts from when you first enter the country. If you take and pass the driver test during that time (both parts) do you get a "regular" license, or would it be a provisional one (the big "L" learner, person with 3 years in the car, etc.)?
You're not misunderstanding anything - you're completely correct
She will have 12 months to drive on her US licence from the date she moves to the UK. If she wishes to drive after the 12 months, she will need a full UK licence. If she doesn't pass the test and get the licence by the time the 12 months is up, she will be treated as a learner driver (must always be accompanied in the car by someone over 21 who has held a UK licence for a minimum of 3 years, no motorway driving allowed, and she must display red L plates on the car).
So, it's a very good idea for her to start preparing for the UK licence as soon as she can, as it can take a few months to get the licence, and she would be wise to take a few lessons to, to learn how to pass the test.
She will need to apply for a provisional UK licence in order to take the tests (she has to show it), but during those first 12 months, she can still drive as normal on her US licence.
Is it possible to take the driving part of the drivers test without actually owning a car (say, in a rental)? When I took my test here, I was in a borrowed car (belonged to a friend who drove with me to the DMV). I don't think she'd want to borrow a car from a friend, but she'd probably consider renting one. Automatic, while she's still legal to do so.
Normally, UK teenagers take their test in their driving instructor's car, since not many have their own car. The instructor's car is good, because it's the same vehicle you learned to drive in, so you know how to handle it, and it's set up for people learning to drive (i.e. two sets of pedals, two mirrors etc.).
Is it possible to buy like just one driving lesson (rather than a full package) to get someone to critique your UK driving before you go for the formal test at whatever the equivalent of the DMV is?
I would recommend more than 1 lesson - the UK driving test is very specific, and difficult to pass... she has to know exactly how to turn the wheel, check her mirrors, when to put the hand brake on, which gear to be in at which speed, how to carry out the manoeuvres (i.e. how many inches she has to be from the curb) etc. - it's about learning exactly what to do to pass the test, not learning how to drive.
She can try with just one lesson, but I'd suggest maybe 3-6 lessons and then see how she's doing.
Are the tests a lot harder than California, if anyone's taken them both places?
The tests have about a 45% pass rate, and it's not unheard of to fail a couple of times before you pass.
I'm British, so have only taken the test here as a 17/18-year-old learning to drive, it took me 15 months, 3 test attempts and about
130 hours of lessons to pass the test! My middle brother also took 3 attempts to pass, but I think he got his licence within about 10 months. My youngest brother passed on the first attempt, within 4 months of turning 17.
Edited: because I apparently can't calculate 15 months worth of 2-hour lessons/week