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?