All three major alliances have Portland to London on sale for around $600 round trip for travel departures from late October to May 10. Delta and American offer these discount prices from late October to May 10. United/Star Alliance only lowers fares for April-May departures at time of post. You can even fly at Christmas time with Delta/Virgin Atlantic for $639, although Google Flights shows even lower fares in the $580s if you can find an OTA selling at that price.
American Airlines Portland PDX – Los Angeles LAX – London LHR – LAX (AA 135 77W aircraft) – PDX
I found $565 round trip tickets with American, however, the itinerary involved an overnight in New York.
Delta Airlines Portland PDX – Seattle SEA – London LHR (Virgin Atlantic) – Seattle (Virgin Atlantic) – PDX
United Airlines Portland PDX – San Francisco SFO – London LHR – SFO – PDX
Christmas Season Fares through Momondo.com OTA meta-search
$639 round trip Delta/Virgin Atlantic Dec 19 – Dec 28