According to the Environment Canada:
On Tuesday, the howling tailwinds for Virgin Atlantic Flight 08 from Los Angeles to London helped it achieve a record ground speed of 1290 km/h and arrive a full 48 minutes ahead of schedule!
This incredible flight speed was caused by a very strong jet stream. Jet streams are wind maxima at high altitudes (roughly airplane height or 250mb), where significant winds are present.
As you can see from the graph above, which was taken from a sounding retrieved above New York, a weather balloon measured a wind speed of 231mph. According to Environment Canada:
This “jet stream”, the high-altitude air current along which storms travel, was blowing in a small area – a “jet streak” - at around 370 km/h.
Above is a surface map depicting calm surface conditions across the Continental United States on Tuesday, during the event.