Everytime you enter UK as a "visitor", you can use the US driving license upto 12 months. The clock resets another 12 months the next time you enter as a "visitor".
If you become "resident" you will need a UK licence for driving after 12 months have passed since you last entered UK. Ofcourse, next time you enter as "visitor" the clock resets, and you have another 12 months of driving privilege. The difference lies in whether you enter as a "visitor" or a "resident".
Some countries like Germany and Belgium fully convert US licenses to local EU-EEA licenses. I easily converted my American (Oklahoma) license in Holland, to the pink EU-EEA license over two decades ago. Since normally EU-EEA licenses are fully convertible to UK licences I thought I would not get problems in UK....
However, when I became resident in UK, they refused to convert my European license from Holland, saying it was originally converted from an American one!!! Talk about being nitpicky!!! I felt discriminated, having an EU license (although based on an American one) that was not being converted to a British licence, despite the EU facility that authorises that, and despite a major EU nation easily accepting US licenses.
Anyway, now I too have to do the driving test in UK...and relearn little things that experienced drivers do away with, but are required for the test.
Take care y'all!!!