Southeastern trains don't have seat reservations at all. Hop on and sit wherever you want.

Same thing for a lot of British trains. On others reservations are optional and most importantly free of charge. Never pay anything for them.

Use the GWR website as suggested here: https://www.seat61.com/interrail-and-eurail-reservations.htm

Thank you very much. :)

So I dont have to add to my pass?  The train I want to take.  How does it work at the staion or on the train if inspector comes along?

Every train you take should be registered/added to the pass, otherwise you risk a fine for fare dodging or you will be refused entry at the ticket gates in the station.

thank you.