Got it. I found an article on how to book open jaw flights using Google Matrix ITA and Hipmunk.
Thing is, cheapest I can get for SFO > ARN and LHR > SFO is $4k on Swiss Air.
It looks like it's cheaper to fly from SFO to ARN round trip on SAS (via CPH) for $2,600 in J, and then maybe just buy Y class round trip from ARN to LHR for the London stay for around $150.
But I'm not familiar with J class on SAS, so I'll have to look into that.