there is a very simple answer about travelling in britain by train, DONT! The train service is absolute garbage! I should know, I live over here and would never travel by train. Who are you sailing with? If it is cunard then they have what is called cruise connect, it is a coach service from london (usually heathrow airport) to the dock. See if your cruiseline has something the same.
hope this helps
What do you mean this isn't how they eat onboard?
Queen Elizabeth 2 - Trans-Atlantic 2001
Queen Mary 2 - Caribbean - 2004
Queen Mary 2 - Mediterranean - 2004
QE2 40th Anniversary Round Britain Cruise.