I plan to travel from CAK (Akron/Canton OH) to LHR (London). I found a flight with a 2h 18m layover in ATL. Is that a long enough layover for an international flight?
yes.
More than enough. I hub out of Atlanta all the time. An even hour works fine. Much under that and I'd worry about checked baggage making the connection outbound. I say this since, on short connections coming home (about the only time I check things) the luggage shows up at the house a couple of flights later.
2 hours is perfect for ATL. One hour is not enough, 1 1/2 is tight but 2 will be stress free.