We find two hours more than adequate to get through immigration, customs, recheck any luggage, go through security and get to the required gate. This applies to Atlanta, Newark, JFK. Miami, Cincinnati, Washington Dulles and Chicago.
If you have through tickets and you miss the connection the airline will get you on the next flight.
On the way back there are no formalities at the second airport and your luggage goes straight through to the UK so an hour connecting time is all we usually allow.
ford family