I just starting taking lessons a few weeks ago. I drove a fair bit my first year here on my US license, but stopped when my license expired (most trips we do are on the motorway, so I can't keep up the practice on just my provisional license).
Currently, I feel 90% comfortable driving in the UK. My instructor said that I do have control of the car and my year's experience shows, however I still needed about 10-12 lessons just to learn to pass the test.
I think being so comfortable driving is really a bad thing, because my instincts tend to take over and I do things that would be considered a major fault. In my lesson yesterday, I had to pull to the side of the road to let a police officer on a call pass. Unfortunately, all the cars behind me decided to overtake as well (rush hour+leaner plates= impatient drivers), and instead of waiting for someone to come to a complete stop to let me in, I just waited for a comfortable gap. It's the sort of thing I would have done on the test, and not given it a second thought.