Well purely as a theoretical exercise...
Assuming OAT of -56c M 1.0 would equal 572kts
A cruise of M 0.92 (I've seen 0.925 quoted as max) would be 526kts TAS
JFK-LHR is 2996NM (3000 for cash!)
Assuming 526kts TAS from takeoff to touch down and an average tailwind of 100kts it should take 4.8 hrs.
Given that we were seeing winds of over 160kts at altitude this weekend and in the past I've seen 210kts+, sub 5hrs looks possible.