easy? well, expensive at least.
even for London - Cologne it is difficult (can't find cologne in SNCF system, have to choose Cologne HBF!) and the fare - unbelievable 189 Pounds! Same day travel from London to ANY Belgian Station is 29,50! A ticket from the Belgian border to Cologne is maybe 10 pounds...
It is really time for the railways to enter the major destinations into amadeus or sabre system, so that connecting fares can be constructed and are available worldwide in a system that automatically offers all the possibilities...