There is something very civilised about catching the Tube to Kings Cross/St Pancras station for the Eurostar journey between the English Capital and French capital. One moment you are using pounds, speaking English and enjoying a Cornish pasty and the next you are whizzing under the water emerging 2 hours 16 minutes later in Paris, my favourite city in the world ready to start speaking French, using Euros and hunting for the best Croissants!
Eurostar is offering a £58 ($US84) round trip from London St Pancras or Ebbsfleet or Ashford to Paris, Brussels & Lille in Standard Class. This is a non-refundable fare. You can take two bags and check in 30 minutes before (I know because one trip I arrived 33 minutes before!). This beats most air fares and the convenience of city to city rail service is extraordinary.