the British refusal to acknowledge the uniquely American Spencerian form of writing, and their dominance over interpretations of WWII for the latter half of the 20th century quite interesting...
I don't think it's a case of a British refusal per se. I think it was a refusal of Edward Johnston, his students and followers (who could and cannot speak for all British people) to adequately acknowledge and/or understand calligraphy and handwriting past the medieval period including British (e.g secretary hand, roundhand), European (e.g coulee, French roundhand) and American (Spencerian) calligraphy.
In addition, Johnston's students/followers were/are not united in how they present the history of calligraphy and the claims they make. Some of them acknowledged that calligraphy continued existing between medieval manuscripts and Johnston, some of them actually claimed that it stopped existing(!).
The society of scribes and illuminators was set up in the 1920s by students of Johnston and still exists today. If you go to their website and look at their about page you can see that the societies conception of calligraphy jumps straight from medieval manuscripts being the most perfect example of calligraphy to Edward Johnston in the 20th century, and it is claimed that Edward Johnston is the father of all calligraphy today.
I think it ultimately comes down to one group of people (who may not be that united or coherent a group anyway) putting their own spin on the past in order to serve their agenda (medieval manuscripts and Edward Johnston). Once false or biased claims enter the written record, it can be very difficult to debunk them and stamp them out. It takes moments to make a claim such as "Engravers invented Spencerian script. It is not true calligraphy", but a lot more effort to debunk it (at least before the internet made so many resources available).
Another thing that occurs to me are that 18th and 19th century calligraphy was often driven by commercial and mercantile concerns e.g business colleges, clerkship, show-card writing etc, whereas Johnston's followers probably thought of themselves as fulfilling a fantasy of a medieval craftsman (thus the title 'scribe' as opposed to 'penman' or 'engrosser') so they would probably react against and scorn things with commercial and mercantile connections such as roundhand or Spencerian.