To those complaining about the thread existing "in a forum for military aircrew" and it's all been talked to death, let me ask/add:
Don't Guardsmen, when on operational duty to two-way firing ranges, fly on military transport aircraft? Therefore, the sentry in question either has been or could be on such a military aircraft?
Sorted, I think you call it.
And I've learned all sorts of interesting old and current nuggets about British military justice dispensation (no comment on it; doesn't sound too different than US, with some exceptions).