I noticed today at my local unstaffed station that the ticket machines now allow you to purchase any UK Rail Rover or Ranger Ticket, whether or not it is valid in your area. I was investigating what was on where (and yes every UK Rover or Ranger is in there, including the UK all line Rover).
This brand new functionality also includes Brit Rail passes. At the moment only the fares for the regional passes are loaded, not for the National ones.
Doubtless that is a matter of time.
But it is worth being aware that Brit Rail does not have to be pre purchased, in digital form. But can be purchased 'on demand' as paper tickets.