I think it’s a trend since there were the faked photos of British soldiers in the mirror for TV shows to now use fictional or defunct military regiments or police forces.
It reduces people claiming TV/Movie footage is real when it’s part of a show. Second season of vigil was similar where people weren’t wearing the RAF cap badge (but that might also because it wasn’t supported by the MoD & had shocking continuity & understanding of the military)
I suspect it’s more of a nod to Conan Doyle’s original material; but, yes, the made up regiments and police forces, changes to ranks and uniforms, etc, provide another layer of entertainment for those with knowledge of these things and an eye for detail.
I second this - it was purposely “Air Force” and never “Royal Air Force” to offer the writers artist licence on what could happen/how people looked as they didn’t need to be accurate