Sales are a bit more predictable ex-UK than ex-EU. Last year I think the summer sale came in May and was around £2k for West Coast, £1.5k for East Coast.

Ex-EU you'll probably have more luck looking for the best starting than the best sale, if you don't mind repositioning (I assume it doesn't matter on the return). Currently LAX looks cheapest from BRU, LUX, OSL; and PRG looks cheapest for your return. LUX-LHR-LAX//JFK-LCY//LHR-PRG is £1458.