I agree with the poster who recommended you look at VS. That is a vastly, vastly superior product to either of the others. The KLM seats, well, they just suck. And on DL you're packed 6-across in a 767 so the seats are only slightly roomier than coach, though with better pitch.

VS on the other hand are roomy, comfortable, make into a real bed, have fantastic IFE, and the Virgin Clubhouse in LHR really negates all the bad about that airport.

I would not hesitate to take VS on that trip for one second.