London to Rennes by train

Something to look out for end-to-end journey times of around 5hrs mins all allow less than 1hr 15mins to make the transfer in Paris from the Gare Du Nord to the Gare Montparnasse.

So targeting the journey options with an end-to-end time of longer of more than 5hrs can be a good idea.
In the rare event of the Eurostar arriving in Paris 20 - 40 minutes late, making those fastest connections into the TGV on to Rennes will become a race against time!

An option is to split the booking into London to Paris and Paris to Rennes journeys so that you can set a connecting time that suits you – though doing so will be more expensive.
See the 'Ticket Information' below.

Or an option is to target the alternative journey options involve changing trains in Lille (**currently not an option);
The big tick in the box for changing trains in Lille is the avoidance of the awkward transfer across Paris.

Though there are also three ticks in the box for choosing this routing via Paris:
(1) More departures to choose from
(2) Tends to be cheaper
(3) The end-to-end journey time is quicker

Book Early And Save: Yes

Online bookings open: 90 days ahead

The very cheapest tickets tend to sell out particularly quickly on this route - they're more likely to be available if you can travel on a Tuesday - Thursday, or are happy to set off from London early in the morning.

The price below is for a one way fare - if you're booking a return ticket, it's likely you will be offered a cheaper price in each direction.

If you do want to split the booking, then and Trainline will sell both the London to Paris and Paris to Rennes journeys.
Add each journey to the basket and you can pay in one transaction.

A plus of booking with Eurostar.com is that you can access a 'Manage Your Booking' facility which enables you to change the seat on the Eurostar, that you will have been automatically assigned when making a booking.
Click on the Eurostar 'train guides’ for more information on how to do this.

The ticketing terms:

If you book Standard or Standard Premier tickets, you can exchange them if need be, to an alternative departure on your booked route; but if you want to transfer your ticket to a new departure less than 7 days before your travel date, you will need to pay an admin fee of £30 for Standard Class tickets, £40 for Standard Premier tickets PLUS the price difference with the new ticket you will be booking.
What's new, in response to the pandemic, is that these £30/40 charges aren't applied if you want to transfer to an alternative departure more than seven days before your travel date.

Business Premier tickets are less restrictive, in response to the pandemic they can now be refunded and they can be exchanged to alternative departures without paying the admin fee, or the price difference.
Eurostar's full ticket terms and conditions are here

Using Rail Passes on Eurostar:
Rail pass users can now pay reservation fees to travel on Eurostar trains, in a similar manner to other international European high-speed trains.
The new rail pass reservation fees are similar to the cheapest single journey ticket prices, but as those cheapest tickets can sell out quickly, it's likely that the rail pass reservation fees will save money.
You can purchase these online with no booking fees via B-Europe - or they ARE now available on the Eurostar website.

Booking in advance is recommended as it's not unknown for rail pass reservations on Eurostar services to sell out days or weeks in advance, there is evidently limited availability per departure.
You can obtain a rail pass reservations without paying a booking fee at Eurostar stations.
However, if you want to do this on the day of travel be prepared to be flexible re: the train you will take.

If you are a UK resident, your rail pass + reservation is only valid if you take a train from the UK on the first day that your pass is valid for and return to the UK on the final day that you will be using a pass.

This text refers to the pre-pandemic timetable

The late morning train from Lille to Rennes used to have a well-timed connection with a Eurostar from London, but it now leaves Lille at 11:25 - only two minutes before a train to London is due to arrive!

The 15:04 Eurostar from London arrives in Lille only 13 minutes before a train to Rennes will be departing at 17:39.
This is evidently a connection that's too tightly timed for the ticket agents to offer this combination of trains as an end-to-end journey.
You could gamble on booking separate tickets for these London to Lille and Lille to Rennes journeys, but in that scenario you will have to re-book the journey to Rennes if the Eurostar is delayed.

The other possibility for making a London to Rennes journey via Lille involves taking the Eurostar, which departs London daily at 12:58.
There is a daily TGV from Lille Europe station, which is usually scheduled to depart at 17:39, but the Eurostar from London is due to arrive in Lille at 15:30.

As the waiting time in Lille between trains will be more than two hours, the ticket agents also won't offer this combination of trains.

But if you would prefer to spend just over two hours in Lille than make the transfer between the Gare du Nord and the Gare Montparnasse in Paris, then an option is to book separate tickets for the London to Lille and Lille to Rennes journeys.

The transfer time of two hours means that a missed connection due to a delay, is a very unlikely scenario.

The journey time shown below is for the journey option with the two hour connection in Lille.

Book Early And Save: Yes

Online bookings open: 90 days ahead

The search results for the London to Rennes journeys on Eurostar combine this journey via Lille with the alternative connections via Paris.

The journey option via which can USUALLY be booked on the Eurostar website is the daily departure at 15:04 **this departure is currently suspended).

What is odd is that very similarly timed connections in Lille are available when departing from London at 08:55, but neither the Eurostar website, or any other ticket booking site, seems to offer that journey option for an end-to-end booking.

Though if this would be your preference click here for our guide to the London to Lille journey and here for the guide to the Lille to Rennes journey.

If you miss the connection in Lille:

If the 15:04 Eurostar arrives too late in Lille to make the connection, then the only option for reaching Rennes by the end of the day, is to travel via Paris

On this route, there is little point in booking separate tickets for the London to Lille and Lille to Rennes journeys, so that you can extend the connecting time in Lille between trains.
All of those alternative options involve having to wait in Lille for more than two hours between trains.