Groan. -) This is another one of those "specialized usage vs general consistency" dilemmas that comes up so often. Is WP a general encyclopedia that includes philatelic material, or a union of general and specialized encyclopedias? Secondly, is it a worthwhile effort to fight for a special exception to WP's usual practices? I would say not, because industry usage seems like more of a longstanding convention rather than a key point of fact (I've never seen an "official" explanation of the GB preference, for instance), and redirects work well enough to get people to our content (there should be category redirects irrespective of the name chosen). Deletion review or categories for discussion would be the right places to reopen, but based on past experience, "everybody in philately uses GB" alone is not going to be compelling enough to get an overturn.