A film like TUNES OF GLORY is made in four stages:

(1) SET UP this is when the producers dial in the script and attach the various performers, directors, camera, and craftsmen to the film project.
(2) PRE-PRODUCTION puts everyone on the payroll, builds the sets, sorts the costumes rehearses the cast, and takes care of any last minute glitches.
(3) PRODUCTION is all of the time spent in front of the cameras, which can be anything from a few weeks to several years depending on the complexity of the story.
(4) POST-PRODUCTION in it's simplest terms means editing the film footage into a cohesive narrative (in other words making sure the picture has a logical beginning, middle, and end).

The entire process of making a film is constrained by two things: the total budget the studio has allotted the producer for the project, and the amount of time available to the producer to complete steps 2-4. Film making is not a leisurely process, and frankly on a small budget film like Tunes of Glory the producer probably wouldn't have wasted time or money on a special weave of tartan-- his costume designer would have selected something readily available, checked with the set designer and the director of photography to make sure the fabric would "work" on camera, and would then schedule fittings for everyone who would be kilted in the film.

What the costume designer wouldn't have done was sit down, design a tartan, have it woven, then have kilts made.

So, if it's the consensus of well-informed opinion on XMTS that the regiment is wearing brown Scott tartan-- well, that's good enough for me.