Remember the old POSH rule (Post Out Starboard Home) from shipping days of going from Britain to Asia, where this kept you on the north (or shady) side of the ship both ways.
It's still applicable to flying because it prevents you getting the glare of the sun reducing your view. It's particularly significant if you want to take photographs out of the window. And it applies to your transatlantic sectors as you are starting from the US and coming to Europe.
You obviously have to give consideration though to which way you are going to keep on the opposite side from the sun. If starting from Britain and going transatlantic it's pretty much SOPH, although if you want the best view of parts of the US at the end of the trip and it's going to be afternoon when you get there, the opposite side can be better.