UK rail & Eurostar
Travelling from around the UK to St Pancras International to take a Eurostar to Europe
National Rail offers flexible fares for travel to London or Ashford to connect with a Eurostar departure to Europe. These are separate tickets which should be used in conjunction with your international travel ticket. You will need to be able to produce evidence of your travel with Eurostar, eg an email confirmation.
Tickets issued to "London International for Eurostar" have fewer time restrictions (often none at all) compared with normal tickets and they ensure "Contract for International Carriage of Passengers and Luggage by Rail (CIV) - PDF" apply to your whole journey, rather than just in Europe. This protects you in case one of your journeys is delayed resulting in a missed connection.
Tickets to "London International" also include the London Underground across London (if necessary) to St Pancras.