Closer examination of WOW Airlines sale fares shows San Francisco to London or Paris is $269 round trip on some dates Sep-Oct 2017. Amsterdam is as low as $300 round trip.
WOW Airlines San Francisco SFO – Paris CDG
$268.99 round trip Wed Sep 25 – Thu Oct 5
WOW Airlines San Francisco SFO – London LGW
$268.99 round trip Fri Sep 22 – Sun Oct 1
Carryon bag is $40 each way and checked bag is $60 each way.
Cheap seat assignments in a 2-seat aircraft row section in the rear of the cabin are available for the SFO-Iceland segment for $10.99 each way. I flew in one of these back of the WOW aircraft seats this year and enjoyed it.
Iceland to London or Paris seats are $6.99 or $7.99 each way for $36 to $38 total round trip for self-selected seat assignments. Flights I checked had plenty of aisle and window seats availability.
WOW Airlines San Francisco SFO – Paris CDG
$385.93 round trip Wed Sep 25 – Thu Oct 5
Includes one carryon bag and self-selected seat assignments.
Iceland Stopover
Iceland stopover tickets for London or Paris are as low as $396 round trip, but adding an Iceland stopover increases the cost of bag fees.
Carry on bag increases from $80 to $110 for Iceland stopover round trip ticket.
Checked bag increases from $120 to $170 for Iceland stopover round trip ticket.
WOW Airlines Iceland Stopover ticket $395.99 round trip
Sat Sep 23 San Francisco SFO – Keflavik, Iceland KEF (3 night Stopover)
Wed Sep 27 Keflavik KEF – London LGW (6 nights)
Tue Oct 3 London LGW – KEF, Iceland – San Francisco SFO
Add $150 for one carryon bag and seats for $545 round trip to Iceland and London.
I flew WOW Air to Europe twice this year for $70 one way with only free personal item and no paid seats. Both trips went well without being stuck in a middle seat for the transatlantic flight to Iceland.