At face value, US and UK driving tests look similar - written (theory) tests with multiple-choice answers, followed by a road test where an examiner sits beside the trainee driver and deducts points for minor errors and immediately fails the candidate for major errors. The only difference in this basic description is that the UK now includes a third test - reactions/hazard perception. On paper, therefore, the tests look similar.
But they are not...as the low pass rate for the British test (around 43%) shows.
One principal difference is the length of the practical driving test (45 minutes). That's quite a long time for a novice driver, under the stress of test conditions, to drive in a manner that demonstrates competence with no significant errors. Also, the test will include major highways (there may be some exceptions to this, I admit - a test centre on the Scottish islands or in rural Wales perhaps) - but if you take your test in any large British town or city, you will end up in heavy traffic, on busy highways.
The examiners are very strict. They are fanatical about proper observation. Simply put, it is very easy to lose points and fail your test. Everyone in Britain could probably regale you with stories about how they, or a friend or relative, failed. My nephew failed because he indicated too early leaving a multi-lane roundabout, realized his mistake, and turned off his signal before waiting for the exit the examiner had told him to use, and signaled again. Instant fail (classified as "dangerous" - (had he just left at the exit he signaled for, the examiner may have been annoyed at having to adapt to a change in route, but technically it would not have been a fail). Yellow-box junctions are a favouite, too (these control right hand turns at busy intersections, the equivalent to a left hand turn in the states - the idea being that they enable drivers to drive up into the intersection and wait to turn in instances where there is no turn arrow). I have known many people who failed their test because of yellow box junctions. Too timid to pull into the intersection at the proper time? You fail. Too bold, and you enter the intersection before your exit road is fully clear? You fail.
There are three maneuvers (turn in the road, parallel park, reverse round a corner). These are performed on the road, in traffic - not in a parking lot. Again, the examiners are strict. Bump the kerb however slightly...fail - although most people who fail on the maneuvers fail for inadequate observation.
There is also the usual stuff - hill starts, emergency stops. However, the test now includes an "independent driving" section. Driving a route shown to you by the examiner, without waiting for his instructions to, "turn left at the next light," etc. You don't fail for minor mistakes in the route, but the idea is that independent driving shows how people genuinely drive (especially in terms of lane changing, etc), when they are driving to accomplish a journey they have in mind - a different mindset to an instruction to make a particular turn, delivered in a timely manner that allows the candidate to respond to it in a formulaic way.
If you pass your test in an automatic in the UK, your licence is endorsed "automatic only." You'd have to pass the test again in a manual car to be allowed to drive a manual.
As I said, on paper the UK and US test look very similar. The difference, really, is in how the tests are administered. Teenaged learner drivers complain that the government is picking on them, doesn't want them to drive, the tests are hard so the government can make money from the test fees, that old people are the real danger and they should be tested too, it's just learning to pass a test and you'll probably drive differently once you've passed, etc etc etc - all the usual stuff I probably also said when I was 17 and learning to drive. The aim, however, was to reduce carnage amongst young drivers, and this seems to have worked, to an extent (accident rates for teens are falling). That said, the UK can learn from the US, too. The next idea is graduated licensing (as, I believe, is used in Maryland), with restrictions placed upon the first year or two of driving: curfews, limits on the number of people a new driver can have in a car, and a lower toleration for penalty points before a licence is revoked and the driver must be retested.
All a bit wordy, I know, but you asked so I tried to answer your question.
Here is something interesting though, a short article by an American grad student studying in the UK, who recently took a UK driving test, and was surprised at the differences between driving tests in the two countries: http://www.theguardian.com/commentisfree/2013/dec/02/uk-driving-law-versus-us