A few lessons is a good idea, in the town that you'll have your driving test in. I had 2 or 3, which was enough, but they were _critical_ to learn things like where and when to check mirrors the UK way, backing around a corner, etc.
I got a CD that prepped me for the theory part. I cannot recommend it highly enough. Without it, I would have failed. With it, the test was an absolute breeze. Practice both the written questions and the video where you click when you see hazards.
As for your US drivers license, yes, you keep that.
Good luck!