You can’t book a cash flight from the U.S. to the UK directly through the Alaska Airlines website. That’s not to say you can’t earn any Alaska miles on that flight, though.
Even if you can’t earn the 3x miles on your card, you could still earn base and bonus miles in Alaska’s loyalty program via one of Alaska’s partners if you add your Mileage Plan number to your reservation. Partners and terms are listed here: https://www.alaskaair.com/content/mileage-plan/how-to-earn-miles/airline-partners?int=AS_MP_HowtoEarn_-prodID:MPPartners And if you used your Alaska card for that purchase, you’d also earn 1x miles on top of that. It’s not as nice as 3x miles, but it’s something.
(This is kind of tangential to the question you actually asked, but you can also redeem Alaska miles for international flights with Alaska partner airlines through Alaska’s website. So if you happen to have a bunch of Alaska miles, maybe that’s worth considering, too.)
To your point about the acquisition, the airline formerly known as Virgin America is now operating under the Alaska name and logo and is offering service on certain routes Virgin America used to offer. You can book cash or award flights on these routes through Alaska’s website and earn 3x miles on your card. Virgin Atlantic operates separately, though.