I started my service with a 6 digit apprentice number. In 65 I acquired a letter prefix, but it was followed by 0 (zero) to pad it out to 8 characters.
On commissioning the letter was moved to the end and my number usually then had 7 characters, but on some documents it was again padded to 8 by a zero at the front.
Since retiring 16 years ago I have never felt constrained to use my rank although some ex military people will sometimes ask. I don't think it's really relevant anymore. However, it was my understanding that officers of field rank(major) and above could use their rank on retirement if they wished and those below had to apply for permission to do so.
Pedant point: PN it's not the case that Gp Capts and above don't have numbers, just that they are no longer required to be used on lists, etc.