Those prices are what the Interrail's agency will charge you for a reservation.  Your Interrail pass generates your ticket and there is no need for a reservation on most trains which, however, you can make at that price or for free at a ticket office or on some sites - the GWR new look has removed the option to do just-reservations but I believe it's still available at TPE without a fee. 

You DO need to make reservation in the UK on sleeper trains and some open access trains.  There may be the odd one of those in the screen capture I did, but most of those are National Network services which you can just jump on.  Without a seat reservation, you are not guaranteed a seat


Well identified down to the line.  I was pointing out the potential absurdity if all UK trains went "reservation required".

 
The interrail example at those prices must be Advance tickets surely - which require reservations - as do Lumo services which may also be included in the services being offered in that grid.

That photo is of an London Underground service (Bakerloo?).

 
Looking at the GWR booking engine for this Saturday, Melksham to Weymouth:





Me thinks this is incorrect - and yet "reservations compulsory" or "reservations required" flags seem to be coming up at various odd places - this (above) is a GWR example and here is an Interrail example:



With multiple "errors" suggesting reservations are needed, I find myself wondering if there's any truth in the conspiracy theory that the railways want us to reserve seats whenever we travel in the future and are softening us up by plugging a false requirement.

For sure, it might help avoid scenes like this:

but wouldn't it lead to overcrowding at some stations while people waited for the train they were booked on rather than getting the next available service?