The journey by train to Athens (from where you get the ferry to Crete) takes 2 to 2� days from London.
e.g. leaving from St Pancras at 1434 next Wednesday would see you arrive in Athens at 1818 on Friday. (Change at Brussels and Frankfurt, then take the overnight train to Vienna. Then change again at Budapest and Belgrade, from where you get the overnight train to Thessaloniki for a connection to Athens)
From Athens station you'd need to get to the nearby port of Piraeus (probably by taxi), then take a ferry to Heraklion or Chania. Conventional ferries take around 9 hours, with the high-speed services taking around 6 hours. Some of the 9 hour services operate overnight but, if your connections didn't work out, it might be necessary to stay in Athens overnight, and then travel to Crete in the morning.
Lastly, you'd need to arrange local travel (to your accommodation) within Crete. So the whole trip would take around 3 (or possibly 4) days in each direction.
You can plan rail travel anywhere in Europe using the German railways web site:
http://reiseauskunft.bahn.de/bin/query.exe/en
(NB: Enter 'Athens' as 'Athenes')
Deutsche Bahn's UK booking centre is also the best place to check on fares, and to book tickets:
08718 80 80 66
Chris