Are you heading to London, proper? If so, I would see if I could route SEA-AMS-LCY. You minimize the amount of time on domestic equipment, get to try the lounge in AMS, minimize immigration hassles (LCY is much better than LHR in this regard), and it is faster to Central London. Win, friggin, win.

I would still probably go via AMS, even if I had to fly into LHR, if only because the arrival time for the DTW flight is peak arrival time, and I have always found immigration to be tough at this time.