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Legend of the Seas is not a "vast ship" she's a medium size and even she would move about a bit going out of Southampton. Quite honestly the bigger the better as far as motion sickness is concerned. Smaller ships than that would include most of Fred Olsen’s and Saga but if you are going south you've got the Bay of Biscay to contend with and east the Atlantic and North and East the North Sea so you might have to contend with some lumpy water out of an English port. The Millennium class of Celebrity and the Golden Princess range of Princess Cruises, both bigger than Legend and pretty stable as are several of Holland Americas fleet. One thing you might consider, and it is practical, is to get a train to Barcelona, Venice or Rome and then board your ship which could lead to a far smoother passage as you don't want to fly.....Nei.
Try Saga??? BTW Legend of the Seas is not vast - wait till you see Independence of the Seas - now that is seriously vast.
Maybe you should try P&O's new Ventura, if that is too big maybe you should try the smaller ships in the P&O fleet, Aurora or Oriana, they seem to handle the rough sea's very well. Avoid Cunards Queen Victoria, I found it rocky even when the sea was calm. Have you tried a mid-ship cabin on a lower deck?
ARCADIA with the added bonus of no childre.