I have to disagree with the general opinion here.
Although I am BA Gold and read the BA forum (which is really great, btw) regularly, I cannot say that Lufthansa C is that bad at all. I have flown both BA and Lufthansa transatlantic in both C and F recently, and while I agree that the BA lounges are much nicer and the flat bed is a real advantage in C, the food (for the european and the intercont. part) is much much better on Lufthansa flights.
As this flight is supposed to go through Munich and the Munich lounges are much better than the rest, I personally would not sit in Y from ATH to LHR.