I don't think you will find any Southhampton (London) to New York cruises in December. The North Atlantic is quite rough and cold that time of year.
Most of the repositioning cruises from east to west will happen in the fall of 05.
Princess, Royal Caribbean, NCL, Cunard, Seabourn all have London to New York, Trans Atlantic cruises from August thru November. Most of Cunard cruises on the Queen Mary 2 are the regular Trans Atlantics and the other cruises are repositioning cruises.
I would take a look at the cruise lines websites to get an idea of which Trans Atlantic cruises are available.