True but irrelevant
As a maintained Build Standard is a mandatory pre-requisite to (a) a valid Safety Case and, in turn, (b) a valid Release to Service (the Master Airworthiness Reference), may I respectfully suggest that the Build Standard is never irrelevant.
MoD stopped routinely maintaining Build Standards in 1991. There are 17 core components of a BS. I'm pretty sure 95% of DE&S couldn't name 5. Even if they read this and wanted to know more, they'd have difficulty as the only Def Stan that set them out and described how each was to be managed was declared obsolete in 2008. That is my main point - MoD's complete ambivalence to the subject ("a waste of money") and aviation safety in general.