Booking engines sometimes allow you to over-travel to the London Terminal on a Not London ticket to come back 1 stop. Doesn't mean its a valid route.
Fair point, but that's poor customer service from the railway industry as a whole (putting things politely), isn't it? Putt yourself in the position of a passenger who had used a booking engine (which supposedly applies the routeing guide) to buy a ticket for their intended journey only to find out that the ticket they had been sold wasn't valid, depite them doing everything in their power to buy the correct product: wouldn't you be slightly piqued?!

I must be honest here, I rarely buy [full] tickets and have had free travel for most of my life. Be it like now TOCNE, or as a nominee or as a dependant.
But I would also use common sense and see "Not London"/ "Not Underground"/ "Not on a Tuesday on an even month in the lunar cycle which follows an odd month". And realise what it means (obviously the last one virgin are yet to introduce).