After departing Belfast at 20:00 (8 pm), Titanic arrives in Southampton just after midnight. She would be towed to Berth 44. She traveled 577 nautical miles (664 miles) and her recorded maximum speed is 23 1/3 knots. That is approximately 26 miles per hour.
Sources:
Books
Behe, George TITANIC: SAFETY, SPEED AND SACRIFICE, Transportation Trails, Polo, IL 1997
Eaton John P. & Haas Charles, TITANIC TRIUMPH AND TRAGEDY, SECOND EDITION, W.W. Norton & Company, New York, New York, 1995 First American Edition
Lord, Walter, A NIGHT TO REMEMBER, Holt Rinehart and Winston, New York, New York, 1955. Multiple revisions and reprints, notably Illustrated editions (1976,1977,1978 etc)
Lord, Walter, THE NIGHT LIVES ON, Willian Morrow and Company, New York, New York, 1986 (First Edition)
Lynch, Don & Marshall Ken, TITANIC AN ILLUSTRATED HISTORY, Madison Press Books, Toronto, Ontario Canada, 1992
Internet