Because of what they see as the restricted availability of collectables these collectors think that they should not be listed in the (Stanley Gibbons) catalogues. I must admit I have some sympathy with these views, but of course there are many factors which make their inclusion mandatory.
1. Because the stamps exist and are distributed by Royal Mail's philatelic bureau, then a significant group of collectors, especially those outside the UK, expect them to be included in the catalogue.
2. Many collectors use pre-printed albums, of which there are several brands, each with its own scope. It's to be expected that if stamps were not in their catalogue they would not be in SG's pre-printed albums. If not, then collectors who had these stamps would probably buy a different pre-printed album.
3. But the most important factor is the Catalogue Editor's criterion for listing:
"They must be issued by a legitimate postal authority, recognised by the government concerned, and valid for postal use... Stamps must be available to the general public, in reasonable quantities without any artificial restrictions being imposed on their distribution."And therein lies the rub.
With the separation of the service provider and stamp issuer (Royal Mail) from the retail network (Post Office) the stamps are generally available, with no restrictions imposed by Royal Mail.
That the branch network has been transformed so that receiving letters and parcels is a very minor part of the activities of even the Post Office part of many branches (just look at postoffice.co.uk and see the full range), and because Royal Mail do not pay sufficient commission to Post Office Ltd branches to sell their collectables, special stamp products get a very low profile at retail level despite all the efforts of Royal Mail marketing, including social media, which is really aimed at attracting new customers to the Stamps and Collectables business at Edinburgh.