A few people just starting out - fun! Here was my experience:
I'm 37 and have been driving for 21 years in the US (Los Angeles). I originally learned how to drive on a manual, but have been driving automatic for 20 years.
I moved to UK in September, and bought a car in the beginning of April - an automatic. Had never driven here, but had been a passenger enough to have observed the 'flow of traffic'. Decided to just hop in and drive.
It was white knuckles for the first couple of days. I stuck to driving around my little town, and a little motorway driving - just to get used to things. After the first couple of days, I started to relax a little, and became much more used to driving around - and a lot more comfortable.
After driving for a little more than a month, I went ahead and got my provisional license, and took the theory test sometime in June. Then, I booked my practical for about six and half weeks later.
At this point I started looking for an instructor, with a mind that I would only take a couple lessons in order to understand the 'standards' for the test.
One thing I can definitely say worked in my favor is that I was already quite comfortable behind the wheel after having driven for over two months in the UK. So, all the silly little things they make us do were a little easier to cope with since I didn't also have to deal with 'learning to drive' in the UK.
So, I ended up being able to fit three lessons in before my practical test. Learned all the silliness like turning the steering wheel, applying the handbrake before shifting from first to reverse and vice versa, checking the mirrors before signally, making sure to get in the 'perfect' position on roundabouts, reversing around a corner, reversing into a parking bay, etc. etc..
Went to my practical test feeling pretty confident. I failed the first time around -but for a pretty lame reason in my opinion. I had only five minor faults - but one major fault. Apparently my 'emergency stop' was not 'emergency' enough for the examiner. So be it. Took my second practical test two weeks later and passed.
So, based on my experience, here's some advice I can offer:
If you've been driving for a long time in the US, driving here is not that different - really.
If you can be a passenger in a car and observe how traffic flows - do it. It helps a lot. My 'passenger' experience was only in taxis now and then - but I was paying close attention each time. And a short cab ride is cheaper that a lesson!
If you've been in the UK less than a year, and therefore can still drive here on your license, just do it. Be cautious of course. Know that it will be nerve wracking at first, but it will pass. I started driving with one golden piece of information - always yield to traffic from the right, traffic from the left yields to you.
Drive for a while to get comfortable before taking lessons. Adding all the 'little things' to your routine are much easier if you're already comfortable behind the wheel.
Get an instructor that's already had experience with someone like you. In other words, they've already prepped several people for the practical test that have had many years of driving experience in the US or elsewhere. My instructor knew exactly what I needed, and the stuff I didn't, and it went much smoother because of it.
Have confidence that your years of experience driving in the US are absolutely relevant here, and you'll soon see that for yourself. Maybe this will help you relax a little.
That's my quick two cents. I remember when I started out - reading this thread made me really nervous. Perhaps the more people that post their experiences, the more everyone will see that the experience of getting a license here doesn't always have to be such a major pain.