I fly this route quite often and am usually on 940. However, because I need to be in London next week with no flexibility, I did book the 850p flight just to give myself options of two later flights if my flight is impacted. I don't like flying on the 5/6p departures as it's too early for me to sleep and landing at midnight ET isn't ideal.

You're going to drive yourself crazy checking daily, as whatever happens today, tomorrow, etc. has no bearing on your flight. I will say that the 11p flight is the only one I avoid - I've seen it multiple times where they swap an aircraft that has an MX issue, in hopes it'll be resolved before departure, only to see it cancel. A reddit post from a UA employee over a year ago also singled out the 11p EWR-LHR as one of the worst.

My advice - start tracking your flight once you have the tail # and look for delays. Until then, don't pay attention to the daily delays. Very few international flights are canceled and international flights are prioritized over domestic, so you're more than likely going to be fine. But, of course, anything can happen... especially with the old 767s.