I really don't think any of us here as reasonable historians are quite that far apart in our views that art and for that matter most sources of data and record of historical events may not always be entirely accurate. While it seems clear that the main issue is trying to establish the motivations and perspective which might lead to any inaccuracy or deliberate nuance or 'spin' on the source, obviously these are opinions, and should be accepted as such.
I try to make it clear when I am presenting material that my comments are my opinion or view, not necessarily a matter of irreproachable fact. The only time I can assert otherwise is if I actually witnessed the event.
Ironically, as most law enforcement and investigative authorities will tell you, even eye witnesses are often not entirely accurate, thus they rely on cross investigation to compare details.
Ariel, thank you for your comment on my story, which indeed I meant in that very way, to be instructive.
A note regarding Wikipedia, which I confess to using a great deal. It is what I refer to as a 'benchmark' source, presenting material on a topic and hopefully with cited sources. From here the research BEGINS as these are checked and cross checked with others as networking through the materials moves forward.
Actually, art and Wikipedia in this sense have this in common!
Actually the Wikipedia entry concerning the ghastly practice of execution by tying to cannon muzzle and firing seems well covered, and the pages of cited references and extensive bibliography have well set the path for any residual research. It does seem the practice certainly was not British alone just as most cases of these kinds of grim circumstance are not restricted to any particular nationality or other denomination.