A number of delayed flights leaving JFK airport still managed to arrive in LHR earlier than scheduled. Several New York to London flights had a flight time of 5 hours and 20 minutes versus the typical 6 plus hours as a result of a powerful 200 mph jet stream.
According to data from Flight Aware, one flight even reached a groundspeed of 745 mph. Although speed of sound at sea level is 761 mph, they do point out that the actual speed was much lower and the plane was traveling within its limits.
Jet stream is typically stronger at this time of year which is great news for those traveling JFK-LHR. Those traveling LHR-JFK, however, should plan on delays.
To read further, including more interesting information on headwind diversions here is an article from mashable.