Originally Posted by
Travelomania
Just a suggestion and FWIW: ... You can avoid the highest departure taxes in the EU - the UK added
Air Passenger Duty Tax - by NOT departing from LHR to return back home and easily save yourself enough to pay for the train ride on the Eurostar

Instead make it an Open Jaw - fly into LHR, but depart from either BRU, CDG, CPH, AMS, FCO or any other neighboring airport on your train route.
Also note that Inter-Europe flights are dirt cheap when booked sometime in advance.
The other airports have their taxes too. At least CDG and AMS do. And CDG has an extra large business and first class tax.
But it is a good suggestion to shop around to see which one is best. And the OP would get to see more of Europe.