Lots of Peaks found their way down there, I used to travel regularly from Sheffield to Birmingham New Street in the early 70's, and the trains went to the west country, (Plymouth, Exeter or even Penzance), or South Wales, and were often class 46-hauled, later 45/0.
Perhaps yours failed at Plymouth, went to Laira for rectification, and was then rostered locally to run it?
"A Prototype for Everything" as the Railway Modeller used to say;)