The GB operators decided a few years ago that their reservation system didn't need to be connected anymore to the rest of Europe (not Brexit-related) so that's why reservations can only be made in GB or by contacting the operator directly. If you're using Avanti Westcoast: reservations are optional and probably free.

If you're only making this one trip, an Interrail is not worth it and you can buy normal tickets e.g. here.

We already own a pass, this is why we are interested in reserving with Interrail. We are coming from France on the same day and therefore cannot buy a ticket before, so is there a guarantee that we will be able to reserve our tickets for free only few hours before the departure ? 

There is no guarantee but since reservation is optional, there a very high chance you can use the train.

If you really want a reservation in advance, please contact the operator directly. If you post the train(s) you'll be using, people can advise which operator it will be. You can also see that yourself on National Rail.

You do NOT need mandatory REServ for the avanti L´pl-Lon. Direct Trains run hourly. There are MANY more connections by changing somewhere, f.e. Crewe or even Nottinghem=last train on Midland brings you right into St. Pancras, same stop as €*. But you likely have to connect into the €* same day, so not much time to dawdle around.

In short: you worry (again, like so many here) for nothing.

@Lucy24 

You can contact Avanti Westcoast via Facebook or email and mention you have a RAilpass and wanna reserve a seat It´s quite a easy process and free of charge in GB.

There are dozens of options every day. Obviously a direct train is preferable but in the hugely unlikely event that you cannot make a reservation you could change at Birmingham, Crewe or even Manchester. 

Safe trip... You'll have no problem