London to Montpellier by train

The Eurostar will arrive in Paris at the Gare Du Nord, but the TGV to Montpellier will depart from the Gare De Lyon.
This guide explains how to make the transfer between the two stations by RER train.
The London to Montpellieer tickets do not include the transfer across Paris.

When making a booking, check the connection time between the arrival at Gare Du Nord and the departure from Gare de Lyon; the wide variations in the current end-to-end journey times are mainly to do with the time between the arrival and departures in Paris.
The current, more limited timetables, have compromised the connections, the majority of which now allow around 2hrs between trains; time enough for something to eat/drink brasserie by the station.

There are typically quicker end-to-end journeys available on Fridays and Sundays, though be wary of cross Paris transfers of less than an hour; they're perfect if the trains are on time, but don't allow for much contingency in the event of a delay; and there are fewer than the usual number of trains, currently available on the Paris to Montpellier route.

USUALLY you can avoid having to make your own way across Paris if you take the alternative journey options via Lille or Marseille, but neither of them are currently an option.

However, there are also 3 ticks in the box for choosing THIS routing via Paris:
(1) More departures to choose from,
(2) you can book the end-to-end journey on one ticket,
(3) and it tends to be cheaper.

Book Early And Save: Yes

Online bookings open: 90 days ahead

The very cheapest tickets tend to sell out particularly quickly on this route  via Paris- 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.

The search results usually combine these journeys via Paris with the alternative connections via Lille or Marseille.
When booking on Eurostar it only gives the option to enter Montpellier as a destination, you cannot look up a journey to either of the specific stations in the city.
Look out for the fact that it' search results also won't distinguish between the stations.

Though a plus of booking on Eurostar.com is that you can access a 'Manage Your Booking' facility - which enables you to change the seat that you will have been initially assigned when making a booking.
Click on the 'train details' for more information.

The different types of ticket summarised

Also worth knowing is that when travelling by Eurostar you can choose between three classes of service, and this also impacts on the exchanges and refunds of what Eurostar refers to as 'Flexible tickets'

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.

Making the connection between the Eurostar from London and the TGV to Montpellier at Lille Europe has the major advantage of avoiding the need to transfer between stations in Paris.

You'll have to make your own way across the French capital if you choose the alternative London to Montpellier train journey option via the French capital.

This guide explains how to make the connection in Lille.

Book early and save: Yes

Online bookings open: 90 days ahead - usually

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, 
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.

The ticket agents listed below will not charge a booking fee when buying tickets for this journey.

The search results for the London to Montpellier journeys on these ticket agents, combine this journey via Lille with the alternative connections via Paris.