Hendon, Duxford and Old Warden would be my suggestions. Those three would cover 95% of the aviation interest in the entire S of UK imho.
If I were to miss one of those it would be Hendon as both Duxford and Old Warden are museums with active aircraft. Rent a car and do those two with an overnight stay in a pub in between...they're not far apart.
But for a visiting American and short of time I'd suggest Old Warden is the one with unique content unavailable in the USA. More or less all the rest can be seen back home.