Apologies if any of this is answered by others.
TAKE DRIVING LESSONS!
Find a Instruction School and take lessons. You know how to drive a car. But you need to learn how to drive a car in the UK. There are subtle differences and a good instructor is well worth the cost to get lessons. As an example, my tendency is to shift my eyes to look in the rear view mirror so that my head doesn't move. My instructor commented that he knew that I was looking in the rear view mirror, but he taught me to move my head so that the Examiner would know that I was checking my mirrors. (I passed first time.) I have since reverted to only moving my eyes, but knowing what to do for the Examiner was critical for passing the test.
Also, if you test on an automatic, you are only licensed for an automatic.
The ratio of automatic to manual here is reverse of the ratio in the states. In the States, 80% of cars are automatics. Here, 80% are manual. If you are comfortable with manual, test on manual.
NARROW CAR:
You want as narrow a car as you are comfortable driving. I was used to a Honda Accord when I moved here. Even a Honda Civic is too wide when I rent a car in the US now. Even if you stay in the cities, the car park spaces are much more narrow than you expect. I ended up with a Honda Jazz, but VW and other brands are well-respected. As you researh models, make a note of how wide the model you're reviewing is (the width should be given in centimeters). You may need to reference manufacture's web sites to get this information, but it will be worth your trouble to do so.
INSURANCE:
You have no driving record in the UK, regardless of your record in the States. So you will be treated as a 16-year old. Accept this. You will pay through the nose regardless.
Half the companies you call will decline to consider you because you have a US license. Keep a list of the companies which will cover you. And then look for the companies which have good reviews for service.
Also, note the major difference: In the UK, the insurance is for the driver/car combination. I am insured when I drive my car, but I am not insured if I drive your car or if you drive my car. Be careful of this difference with the US.
LEASING:
As I had no credit record in the UK, I didn't argue with the system and instead bought a used car for cash. This avoided the whole "getting credit" for a car loan or lease. I would suggest this approach until such time as you have established a credit record. Reasonable late-model cars can be purchased for under £5000.