Scottish Routing Question via Carlisle

[Dewedin]

The yellow pages say Permitted Routes from Edinburgh to Paisley Gilmour Street are all on map FD. Map FD allows travel via Carlisle then the Glasgow South Western Line, but no booking engine will allow it, and the National Rail journey planner says you need more than one ticket to go this way.

The Routing Guide instructions define 'doubling back' as 'passing through the same station twice on a single journey,' so this route does not double back anywhere.

My question is am I right and have I indeed found a way to make whopping savings on extortionate Edinburgh to Carlisle fares?

 

[DaveNewcastle]

Not exactly the same journey, but an Off Peak Return between Carstairs and Carlisle used to be available on the booking engines on several services via Glasgow and even a few via Edinburgh.
Its still bookable via Glasgow, in both directions at £19.80 return, but it now only seems to be bookable via Edinburgh on 2 of the Carstairs-Carlisle leg (and only via Glasgow on the Carlisle-Carstairs leg).
As someone else has pointed out, there doesn't seem to be an easement to permit this, so taking careful copies of all web pages would be advised!
But £19:80 for about 400 miles of scenic travel is interesting!