Travel time between London and other cities in France and Belgium is shorter than using aircraft between them, because of required time getting to or coming back from airports and baggage inspection. But, UK requests passengers coming from Europe by rail to go through passport inspection even when UK is a part of EU.
So, total time required to move between UK and other cities there by air is not much longer than that by train.
In order to operate Eurostar service between UK and Germany, simplifying passport checks and raising maximum speed of Eurostar will be necessary.