To travel by train from the United Kingdom to Belgium, the easiest and fastest option is the direct Eurostar train from London to Brussels. As earlier you book, as cheaper the ticket is.
If the Eurostar train is too expensive for you, have a look at a bus connection from London to Brussels.
To travel by train from the United Kingdom to France you have several options. The fastest one by train is to use the direct Eurostar train from London to Paris. If you search for a cheaper option, have a look at the buses connecting London to Paris. The ferry Dover to Calais isn't an option anymore: too slow, too expensive for foot passengers.
To buy train tickets for your travel from the United Kingdom to France, buy first a ticket from your departure station to London (Trainline UK + ACPRail). A second ticket from London to Paris for the Eurostar train (Trainline + Beurope + ACPRail). And another ticket for France from Paris to your final destination (Trainline).
Of course this example can vary and finally depends on your route.
There are several ferry operators running between the United Kingdom and Ireland/Northern Ireland: Stena Line, Irish Ferries and P&O Ferries. The most frequent services run from Holyhead to Dublin, other connections are available from Fishguard to Rosslare, from Liverpool to Dublin and Belfast and from Cairnryan to Belfast and Larne. Rail&Sail tickets are available, including the ferry as well as the train rides in the UK and Ireland.
To travel from the United Kingdom to India, there is no rail connection available. We recommend to book a direct flight via the given booking link. Train tickets in India are available via 12go.asia.
To travel by train from the United Kingdom to the Netherlands you have interesting options. The main route by train is from London to Brussels by direct Eurostar train. Then by IC or Thalys train to the Netherlands.
If you want it cheaper, have a look at bus connections from London to the Netherlands.
And finally the Rail&Sail tickets from Stenaline on the route London to the Netherlands (ferry Harwich to Hoek van Holland / Dutchflyer) are a good deal - you can even travel overnight (read our blog).