Great Western Coffee Shop

Removed.

Complete rubbish, I'm afraid. Either the person didn't know how, or didn't want to. But an Advantix would let you do an excess for a journey on another TOC, although something like a weekend 1st class upgrade on another TOC might be a different story.

I've bought tickets from a FGW Avantix for a journey entirely on Chiltern Railways.  Sounds like an excuse for 'can't be bothered' or 'don't know how'.

Indeed, I know it's possible, I am more wondering whether it's a contractual obligation under TSA. I can't see an exemption for On-Board.

Avantix should contain all fares, however they may not be available to the operator at first glance. For instance at XC our Avantix is programmed to show all inter-operator tickets and XC only tickets, however tickets restricted to the sole use of another TOC can only be seen after clicking the 'ALL' button, this is to save scrolling through a minefield of other company's Advance and operator specific fares.
In general I think you'll find most guards will be happy to excess a ticket for an onward journey if they have the time, after all they get the commission on the fare and their own TOC will keep the rest of the seller's fee (9% if I recall). However if you're on a busy commuter service or one which is a hot-bed for fare evasion then they may be more concerned with capturing the revenue which will more significantly affect them. (One stop wonders as they're known to me)

Lazy guard.

... which readytostart most certainly is not!  :o

Oh I have my moments!