Very approximately, expect total darkness at this time of the year at Newark moving east to be at about 10PM local time, but take some time off for your eastward travel, so about 2 hours into your flight roughly.
Expect dawn twilight to be around 5AM? local time for the UK, or about 2 hours before your arrival.
Flight should be about 7 hours, so 3 should be in total darkness. I'm sure there are many tables with sunset times, but I use my GPS. Bear in mind, that the GPS will give you these times for the locale of the GPS, perhaps not another location, and those times will be in UTC.
Have a nice flight...