This is something that is discussed all over the internet. Some believe they must buy their tickets when released nearly a year in advance. Others wait until 2-3 months out. (The so called experts once claime the best time is 90 days out for international flights.)
My suggestion is to track the flights you want. If you see a price dip, buy the tickets. Buying tickets when they are first issued does not guarantee the lowest price.
Use Google Flights to track specific flights and to see the cheapest flights on a specific flight for any date you wish.
My personal experience flying across the pond a few times a year is..
--look for midweek flights. Quite often it is cheaper to fly on Tuesday or Wednesday than any other day of the week;
--don't buy December before Christmas. Quite often tickets are bought as Christmas presents so the prices stay high. But look for sales after Christmas.
--sign up for price alerts on sites such as Google Flights;
--regarding the Black Friday, Cyber Monday sales--be careful. Sometimes, what they do, is increase prices prior to these days then drop them back down and advertise them as X% off when you are just paying the same price as a few days before.
--try to travel during non busy times--off season/shoulder season--and never around a holiday.
--sign up for the frequent travel programs for the airlines you may fly. It doesn't cost anything. But they will send you emails regarding upcoming sales.