Were it me, I'd go via DTW on the way to London. But I don't hink you will get the ST lounge on arrival, as it is airside. This is a consideration if you want to get a shower first. Also, as pointed out, LCY is much closer to the city.

On the way back, I'd leave out of LHR, as pointed out, the lounge here is quite nice. The good is good also, so you can eat a good meal if you don't want the Y offerings.

AMS lounges would be open to you as STE+, so SEA - AMS is a real option also. The lounges there are quite nice, but do bring your PM card (even though it is on the boarding pass).

BUT beware, you'll have to change planes in AMS, BUT NOT to schengren area, so will be fairly fast change.

If you go via DTW, as pointed out, you'd get a good meal in the SEA - DTW route. But, if you have certs, I'd call DL see if you can get the upgrade using it.

DAve