I can see Virgin to Portland working. Some background:
Until 2008, when the
Bermuda II Agreement was superseded by the
EU–US Open Skies Agreement, the only US carriers that were allowed to fly into Heathrow were American Airlines and United Airlines (as the successor airlines to TWA and Pan Am, respectively). Other US carriers had to fly to the much less prestigious Gatwick airport. As soon as the EU-US agreement came into force Delta started flying to Heathrow (as did Northwest, with whom it later merged). However given their late start, Delta is playing catch up on the other two big US carriers. As
of last year, American has 13% of the seats out of Heathrow to the US; United 14.34%; and Delta 7.92%. AA is way ahead when you consider that they're in a joint venture with British Airways, who have 31.9% of seats between Heathrow and the US.
This is why Delta acquired 49% of Virgin Atlantic in 2013, which is the maximum a non-EU based company can own. Delta really wants to grow in the UK market, and can't do it all alone. Virgin offer 22.19% of the seats between Heathrow and the US, which means that the combined Virgin-Delta joint venture now offer 30% of the seats between London and the US. Virgin and Delta split the revenues from any flights across the Atlantic to the UK equally, which means that from a financial perspective Delta is just as happy to have you fly on Virgin as they would be if you flew with them.
So why would Virgin consider Portland? Well, Delta is well positioned to sell tickets on the flight, given that they already have contracts in place with large employers in based in Oregon. (Virgin can obviously handle the sales in the UK). BA wouldn't be so well positioned given that its US partner, AA, doesn't have a large presence here. Virgin is also in the process of transitioning its
fleet away from really large planes like the A340 and 747, towards smaller planes like the the A330 and 787. It's probably a lot more realistic to fill 264 seats on a 787 out of Portland than it would be for 367 seats on a 747. Boeing designed the 787 as the plane that could make long, thin routes financially viable for airlines, and PDX-LHR might just be that kind of route.