I’ve flown United Airlines across the Atlantic at least a dozen times for my job, and it’s always been hit-or-miss.

Much of that comes down to the aircraft type: Some of United’s older planes, like its Boeing long-haul 767s and 777s, still have dated cabins and older technology.

But amid booming premium demand, United is working to modernize its fleet and cabin experience from nose to tail, including its aging Boeing 777-200s, which have an average age of about 27 years.

I flew on one of the renovated 777 planes from Newark, New Jersey, to London in July to experience the upgrades. My specific plane was first delivered in 1997, but the modern…