Reporting back on my experience. Couldn't have been better! Signed up at purser's desk as early in the crossing as they would accept (I think Day 3) for "Luggage Storage Southampton", received detailed instructions via letter to stateroom, put out luggage the night before arrival with special provided labels, and was advised in the letter that there was no need to look for my luggage in the terminal. I was able to (indeed was encouraged to) do early self-disembarkation with my remaining bag, and got to the station early. Next saw the luggage when I arrived in my cabin for the trip back to NY two weeks later. Am hoping to do this every time from now on! Wrinkle for next time, however, is that I will be landing Southampton and then reboarding three weeks later at Le Havre to go back to NY. I wonder if my not being on board when the luggage is loaded at Southampton will be a dealbreaker for them.