Global passes include Eurostar. With a global pass you have the right on two of your travel days to exit or gain entry to your country of residence. On a travel day, you can use as many trains as you want.

But you'll need a seat reservation for Eurostar. Just registering the journey in the app is not a reservation. Be quick, they can sell out and there's not lany alternatives. 

More information on how to get the reservations on Eurostar is found here: 

There is a limited number of pass holder seats on the Eurostar between London and mainland Europe. On popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

If you don't get any result, press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

Thanks for the replies very informative I would be going in feb so hopefully not too busy

Some dates in February are already sold out, At least between London and Paris.

If you’re only visiting France consider using point to point tickets unless you’re doing a lot of travelling around. These fares include the reservation fees for TGV trains which interrail does not. SNCF connect app will give you all the fares. Simply add the Eurostar fare. 

Indeed, beter compare prices wisely.

For domestic French Trains, and TGV’s to Germany, Switzerland, Spain, Luxemburg, there’s also a relatively cheap advantage card, which can cut prices quite a lot. With one long trip, it’s usually already payed back. Quite often these Cartes Avantage are sold for 25 EUR. Quite often the better choice over an interrail pass, in France.

More information here: https://www.sncf-connect.com/app/en-en/catalogue/description/carte-avantage-adulte

(Not valid on Eurostar though)

This is great info thanks. I am intending to spend 3-4 weeks in southern and central France with maybe a crossover to San Sebastián in Spain for a few days. so I will be moving about but sometimes using local buses or tours . starting with Eurostar probably then tgv to Marseille so I thought of the global pass but now I'm not sure.

Oh, in that case, you'll have very good value with your global pass!

Based on your comments, including your possible hop to San Sebastián, I would say you have three options:

- Global pass + passholder seat reservations on Eurostar + passholder seat reservations on tgv's

- One-country pass France + separate tickets for Eurostar + passholder seat reservations for TGV

- (no pass) separate tickets for Eurostar + separate tickets for TGV.

The choice between the first two depends on how much your Eurostar tickets would cost you, considering you'll also use a travel day. Be sure to check passholder reservations are still available, as these are limited in quantity (bookable from exactly 6 months ahead).

Since the Eurostar passholder reservation fees are considerable (€35, on TGV they cost either €10 or €20 + probable booking fees), buying separate tickets might be cheaper. A carte avantage can be beneficial as it gives discount and caps the prices (either €39/€59/€79 depending on length), but note the adult version (ages 28-59 iirc) only gives discount for trips involving a weekend.

Hopping to San Sebastián does not require a pass valid in Spain, as you won't be using trains covered by it. Go by TGV to Hendaye (French side of the France-Spain border) and buy a ticket there for the half-hourly Euskotren service direct from Hendaye to San Sebastián (37 mins, €2.75).