There used to be Orient Express trains from London to Athens, but those stopped running... in 1962!
There are at the moment very limited international train services between Greece and the rest of the world. According to Wikipedia, the only trains running at the moment are:
Thessaloniki – Skopje – Belgrade
Thessaloniki – Sofia – Bucharest
Thessaloniki – ...
The Man in Seat 61 has a detailed exploration of train routes from London to Athens, some including ferries.
Note that the state of basically all forms of transportation are in an indefinite state of flux due to the pandemic. It's impossible to say firmly whether cruise ferries from Italy to Greece will be running in December, and if they run, it's ...