Delta is having a fare sale to some European destinations, ending tonight.
I checked JFK to LHR for end of August and returned a RT fare of $558.
Other cities are involved, as well, but I can't say how many because I haven't checked randomly.
The fare from my home to Rome was $914 (down from the $1300 I'd been seeing for weeks for Delta on my selected travel dates). Needless to say, I'm booked for Italy later this year.
If you're looking to buy NOW, look today. The sale looks to cover much of the spring and summer (ending travel on Sept 3 - Sunday of Labor Day). But the email I received telling me of London, Manchester and Edinburgh says the purchase must be completed by 11:59pm EST Feb 10 (today)