OP if it was me Id jump on the non-stop. As others have said the JFK-LHR is a short flight more so if you will be eating on board. Had it been starting in ORD or further West, that would be something else. I usually just cash in for Y when flying NY-Western Europe since its a short flight (almost like JFK-LAX). Not gonna get much sleep anyway so I dont like blowing the miles when I wont be getting my Miles worth

But its a YMMV as each person has their own set of priorities