British seat reservations are optional and free of charge. Rail Planner and interrail.eu are wrong in that regard (the companies have sent wrong info to the European database).

You cannot make them through Interrail. Either go to a ticket office or book through GWR as explained here : https://www.seat61.com/interrail-and-eurail-reservations.htm#UK

interrail.eu is never the best place to book seats (extra fees, no seat selection, etc.). Better to book through the railway company or Rail Europe as explained here : https://www.seat61.com/interrail-and-eurail-reservations.htm

Note that if you live in the UK you must use Interrail.

Thanks for the quick and informative reply. When I select a train with off-site seat reservations requirement, the Eurail app will NOT let me book that train. Without a ticket for that train, I am unable to approach the operator for a seat reservation. I’m stuck in a loop ! Can you further help me with that ? Thanks

What do you mean by "the Eurail app will NOT let me book that train”? The app doesn't book anything, it only registers which trains you (intend to) use.

If you mean you want to add a train to your trip or to your pass, then that should be possible. If not, then please post a screenshot of any error message or of the screen where you get stuck.

There's however no need to activate your pass, add trains to a trip etc. You can simply book seat reservations without doing that. The most you'll need is your mobile pass number.

For Eurostar, book seats on www.raileurope.com. For trains in GB, book via GWR as explained by Seat61.

Sorry - not making myself clear. First time user with a ten day first class pass (x2) that I have only just activated and find we cannot get out of the UK !!!! We are trying to go Chester/ London, London /Paris in April and onwards.  All the direct trains from Chester are Virgin. All need seat resevations,as do EuroStar. Probably a better question is “ How do I turn a planned and saved trip into a ticket that I can use to make a seat reservation with the operator?” Thanks again.

As already explained UK domestic trains DO NOT need reservations, just add the train to your pass and travel, the pass is your ticket and a QR code for each travel day is generated once you have added and activated travel for that day.

If you wish to get a reservation then visit any manned ticket office or use the free seat reservation service on www.gwr.com that is available once you make an account there.

Eurostar needs reserving, the best option is as explained above via raileurope.com.

Have you looked at the steps here?

https://www.interrail.eu/en/interrail-passes/interrail-mobile-pass/getting-started

Please describe the exact action where you get stuck or post a screenshot.

Also, this is completely separate from making reservations. You don't need to have an activated pass, travel day or ticket (QR code) to make a reservation.

The recommendation from the experienced travellers in the community is to wait with activating the pass until just before boarding the first train of your travel. If you activate the pass in advance, and something unexpected happens on your departure day before your departure, then you can't deactivate the pass.