European Train Travel 
There's no need to fly from the UK to anywhere in Europe.  It's  surprisingly easy, quick and comfortable to travel overland by train  from London to just about anywhere:  Spain, Italy, Greece, Prague,  Helsinki, Moscow...
Kick start with Eurostar
Eurostar is the excellent high-speed passenger train from London's St  Pancras station to Paris & Brussels through the Channel Tunnel.   Most journeys to Europe now start with a trip on Eurostar to Paris or  Brussels.  All 1st class fares include complimentary drinks and a meal,  and all passengers have access to a bar car serving drinks and snacks.
Remember that you need to check-in for Eurostar trains at least 30  minutes before departure (10 minutes for passengers with certain types  of premium 1st class ticket, 60 minutes for Eurostars to Avignon or the  French Alps) to allow for an airline-style security check.  
Eurostar  does not run on Christmas Day. 
Changing trains in Paris: 
Train journeys from the UK into Europe often involve a change of train  and station in Paris.  Eurostar arrives at the Gare du Nord, which is a  short walk from the Gare de l'Est but a metro or taxi ride from the  other Paris stations.  The Paris metro website is 
www.ratp.fr.
If you want to spend some time in Paris, by all means take an earlier  
Eurostar on the outward journey or a later one on your return.  There  are left luggage lockers at several Paris rail stations if you need to  leave your luggage somewhere. You can avoid the hassle of crossing Paris  when travelling to many French destinations, by changing at Lille.
Changing trains in Brussels:
Eurostar arrives at the main Brussels station, Bruxelles Midi in  French, Brussel Zuid in Flemish, Brussels South Stn in English, it's all  the same place.  After stepping off the Eurostar, you walk past the  front of the train off the end of the platforms and down the escalators,  exiting the Eurostar terminal onto the main station concourse. All  long-distance trains use Brussels Midi station, including trains to  Amsterdam, Cologne, Paris and all other destinations in Belgium, so  changing trains is easy and only takes a few minutes.