There have been/are some booked to go via the Cathcart Circle, regardless of engineering works, on weekdays to maintain crew knowledge.
Only time I've been on a train up that way which was booked via the Circle, the 1025 Plymouth to Glasgow Central in July 2008, we didn't take it and went the normal way. Mind you, this was on a Sunday evening so could well have been engineering works on or something.
Just because something's shown to be booked via a certain way, doesn't mean it will go that way! Had that problem before on a Voyager booked to go via Pontefract Baghill between Sheffield and York, way back in summer 2006, but we got sent via the through road at Doncaster instead. Still need Moorthorpe to Church Fenton via Pontefract Baghill as it stands, 4 years later!