London-New York area is not just a premium route, it's also a highly competitive route - BA is up against American (obviously as partners), United, Delta, Virgin Atlantic, JetBlue, Norse Atlantic. So you can't look at it in isolation of just BA filling planes. BA has to get the right position in the market, which presumably involves a high concentration of premium seats, frequency, offering the best hard and soft product, offering the right price point, etc. Presumably the A380s, which "feel" important as the biggest plane, don't actually work as the best plane, with all that in mind.

I'd be curious to know what percentage of BA LHR-JFK travellers have an onward connection, especially between Club World, WT+ and WT. I could also envision that BA would much rather push WT passengers through ORD, PHL or DFW for connections.