Okay, so I found this on the DVLA website for people who hold a license from another country:
If you are required to pass a GB driving test in order to gain a full British licence you must first apply for a provisional driving licence. To do this you should complete an application form D1 (previously D750), available from Post Office® branches, and return it to DVLA, Swansea, SA99 1AD with the correct fee.
Once you are in possession of the valid provisional licence, you must comply with the conditions applicable to provisional licence holders. These are printed on the back of the licence itself. You may not take a test unless you are resident in this country. Once you have passed both theory and practical parts of the driving test, you may then apply for your full licence.
So...this is saying I must get a provisional license. And, once I do, that's all I can drive under. I can no longer drive with my US license. So, once I get my provisional license I can only drive with a licensed driver in the car?! Am I understanding correctly? Because that sounds crazy to me.