paula & angie-
this is what I would suggest so that you retain the abiltity to drive without restrictions at all times.
Month 1- Come to UK
Month 6- Start studying for written UK test
Month 6/7??- Apply for Provisional Licence, take written and hazard perception. Start Driving Lessons
Month 11- Pass practical, on road UK test.
Month 12- Your US licence expires, seemlessly replaced by your UK licence.
The problem is if your US licence runs out (over 12 months in the UK), but you don't have a FULL British licence by then. If that's the case, and you only have a provisional licence, you will only be allowed to drive with someone who has had a UK licence for 3 years, not on motorways, etc.
Kind of a hard thing to explain by words, not in person. I think I've spelled it out pretty clear.