On inbound/outbound trains you can take as many trains as you want to get to/from your destinations. There's no limit on the amount of trains you can take or reservations.
Reservations are also completely separate from the pass, no need to book them together. Wise of you, to book Eurostar asap by the way. As it can sell out. That's less of a problem on other trains.
Seat reservations on domestic British trains are free btw, and optional. The best way to get them is via the GWR seat reservation service.
On a travel day, you can board as many trains as you like. In addition, on a travel day with an inbound/outbound journey, you can also travel in your country of residence, of course also in as many trains as you like.
You can make reservations at any time you like. In Great Britain, reservations for day trains are optional, whatever the rail planner app or Interrail website may claim. You can book them free of charge at a GB ticket office, on the LNER website (LNER trains only) or the GWR website (all GB day trains with reservations).
Thank you very much for your answers. I will then make the reservation for the eurostar. Do you know if children under 11 (i.e. 7 and 9 years old) have to pay the reservation? I suppose so, but it’s not clear on the eurostar website reservation. Thank you again :)
The children have to pay the reservation fee. The reservation for pass holder seats can't be made on the Eurostar website. Please read more below.
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
Thank you so much for your answer! Wow it’s a lot a things to know :)
Have a nice evening