Delta is flying PDX-AMS. Portland is a growth region, there's 2.5 million people in the area and there's only that one TATL flight from Portland. Much like the efforts of Hartford to get a route to Dublin, this could be subsidized by an airport in a growing metro area eager to gain a higher profile.

I don't think any such route would be based on UK demand to go to Portland and instead more on funneling Oregonians to the UK. There's no feed really on either side, but of course Portland has a significant business community with Intel and Nike having major presences in the region. Those companies could be pushing for direct flights to the UK (Similar to no doubt the Austin-LHR route for BA is driven by corporate demand).