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Yes if you look up the cunard line they have one way trans atlantic crossings either way.
Look out for ships that are relocating from winter in the Caribbean to the Med in April/May - Fort Lauderdale to Southampton for example.
Yes Cunard or if you are prepared to wait until March and April most of the cruise lines have repositioning cruises from various ports in the States to the UK.
I am going one way to NY in October on the QM2. You just cancel the return flight if it's included.