A manufacturer's mule or shop mule can be anything a company keeps on hand as a fixed asset to aid in their daily operations. It's typically not part of their product line, but rather a necessary accessory, made by and purchased from others, that in some way enables them to do business.
Extreme example: A young woman I knew in the 1980s worked for a major local concert promoter. Her main job was collecting the cheque from the venue on the night of the performance. Another part of her job was, say, collecting Céline Dion at the airport, in style. For both functions, she was afforded the use of the company's shop mule, which was a Guards Red Porsche 944 with black leather interior. I first encountered the term when that very same Porsche was mentioned in an entertainment section newspaper article about the promoter and his company. "Company X is clearly doing all right. Their shop mule is a bright red Porsche 944..." or words to that effect. The term stuck wth me, and I use it at work, to the consternation of some.
All this to say that the term isn't specific to British gunspeak. Its use is far wider-spread.
