How to take the rail journey from Turin / Torino to London

The usual daily schedule will involve:

(1) Taking the 07:11 Frecciarossa 1000 train from Torino, arriving in Paris at 13:22
(2) Taking the 15:13 Eurostar from Paris, arriving in London at 16:30
This is the daily option which involves spending less than two hours in Paris between trains.

On Sundays there is usually an additional optimum connection that also involves spending less than two hours in the French capital

(1) Taking the 13:38 TGV from Torino, arriving in Paris at 19:12
(2) Taking the 21:03 Eurostar from Paris, arriving in London at 22:30

How to make the connection across Paris

Online bookings open: up to 6 months ahead of of the travel date

Eurostar does not sell tickets for this journey.

Seat reservations are automatically included when booking tickets for both trains.

Your ticket will not include the RER train across Paris from the Gare De Lyon to the Gare Du Nord.

Eurostar train ticket terms summary

The only ticket type are the Flexible tickets, but it is the class of service you opt to travel by, which impacts on the terms and conditions of travel.

*Exchanges

Book to travel in Standard and Standard Premier class:
The booked date and time can be exchanged to a different departure to the same destination up to one hour* before departure time.

• If the cost of your new ticket is more than the cost of your original ticket, you will be required to pay the difference;
• If the cost of your new ticket is less than the cost of your original ticket, you will not receive a refund of the difference.

Book to travel in Premier class:
The booked date and time can be exchanged to a different departure up until the end of the previous day, without having to pay any price differences on any other charges.

The Eurostar ticket terms summary is here

Refunds

Book to travel in Standard and Standard Premier class:

• Up to seven days or more before the date of departure you will need to pay a fee of £25 per person for a one-way / single ticket and £50 per person for a two-way / return ticket - So you will only receive any sum you paid in excess of these sums.
• Less than seven days before the date of departure, tickets are non-refundable - Also applies if you exchanged a ticket to an alternative departure, under seven days ahead.

Book to travel in Premier class:
Up until the end of the day before departure date, tickets can be refunded at no additional cost

Seat Reservations on the Eurostar

Reservations are mandatory so seats are assigned when booking - But after a booking has been completed, you use the ‘Manage Your Booking’ facility, which can be easily found on the Eurostar website and select specific seats within the travel class you have chosen.

You will need to enter a Booking Reference number - but other booking agents such as Trainline, Rail Europe, SNCF Connect, B-Europe and NS International, will also include this number on the communication sent to you.

Making the connection in Paris when taking a TGV train:

There is relatively new E.U. legislation which protects travellers against financial loss in the event of missed connections due to train delays, but it only applies to 'through tickets'.
The Eurostar terms and conditions specifically state that when booking journeys which combine Eurostar + Thalys trains 'through tickets' will be issued.
However, 'for all other journeys that combine a Eurostar Service and a service provided by another carrier(s), including when purchased in a single commercial transaction, those tickets will be separate contracts and issues regarding delays, missed connections, cancellations, compensation, and the management of aftersales will be handled accordingly'.

Though both SNCF, which operates the TGV train, and Eurostar are members of The Railteam Alliance which offers a Hop on the next available train (HOTNAT) policy.
It 'allows travelers to take the next high-speed service leaving from the same station as originally planned when a delay on or cancellation of a preceding Railteam member’s high-speed service prevents them from making their originally-planned connection. This service is free of charge and is subject to the following conditions:

• HOTNAT only applies at the station at which the passenger originally planned to change trains and in cases of train delays and train cancellations.
• The approval of HOTNAT is subject to the available capacity on board each train. Seats are not guaranteed'.

So 'issues regarding missed connection and cancellations' should be 'handled' by Railteam's HOTNAT policy.
Though HOTNAT can't be applied if the arrival into Paris is after the final Eurostar of the day has departed for London.

Making the connection in Paris when taking a Frecce train:

Trenitalia, which operates the Frecce train is not a member of Railteam, so its Hop On The Next Train policy does not apply.

Because Eurostar in effect classifies the the tickets for the Frecce train and its train as separate bookings even when booked in one transaction, you can de-risk the need to re-book the Eurostar, at a last-minute more expensive price, by making two separate bookings - for the Frecce train from Milan and a later Eurostar on to London.